Pgylogenetic Studies Of Dreissena Polymorpha

INTRODUCTION

1. Overview

            Human activities can completely remove or weaken natural barriers that serve to maintain and limit the distribution of organismal communities. The opening and construction of canals necessitated the faunal interchange either at the species or genetic level. The results are genetic fusion or hybridization, competitive exclusion of species of inferior taxa or coexistence between the non indigenous and the indigenous species into novel organized assemblages.1 The opening up of the Ponto- Caspian basin marked the beginning of the distribution of zebra mussel from its native endemic ranges to alien habitats.

            All around the world, the introduction of alien exotic species into native ecosystems has resulted in the destruction of the habitat, gradual or spontaneous loss of species diversity and the eventual extinction of indigenous species.2 The introduction Dreissena polymorpha into North American waters signifies the catastrophic loss of species diversity and huge economic losses that the water system and the riparian community, public water and electricity installations has undergone since the colonization of the interconnecting waterways began. Just like all invasive species, their geographical history and patterns of genetic variation has been of deep interests to researchers and policy makers. Data that conclusively details out such concerns are very instrumental in the prediction of future invasions as well as facilitating an understanding into these invasions with respect to the evolutionary context.

            Genetic markers are the most advanced and comparatively reliable methods that can be used to reveal the historical processes that propagated a phenomenon like biological invasions. Ideally, researchers employ phylogenetic methods and population genetic methods in the identification of the taxa involved in the native or non native species biological invasion. Phylogeographic models and multi locus approaches are used to unlock the phenomena behind the cryptic or multiple invasion mechanisms. This information is very useful in resource management measures.3 In research and general practice, various characters have been employed in the field of invasion biology. The direct method, the differential success of invaders or contrarily the differential threat to the native communities have been the benchmarks in studying the morphological, behavioral, ecological life history and the species genetic traits. In this process, neutral markers or in some instances the nearly neutral markers have been behind the revelations of historical processes.

            The success of such markers has been mainly due to the fact that these markers can be hierarchically structured and that the changes in genetic structures of organisms generally change across a time period. Therefore, the genetic architecture of invading species can be analyzed to provide an insight into the invading process as well as its history. This means that phylogenetic studies are essential in identifying genealogical relationships inherent in the participating taxa, for instance, in both the invaders and the invaded species. Genetic studies also have the capacity to infer the invasion history a single individual species as well as that of morphologically similar species.3

1. Scientific Classification

Kingdom: Animalia

Phylum: Mollusca

Class: Bivalvia

Sub Class: Heterodonta

Order: Veneroida

Superfamily: Dreissenoidea

Family: Dreissenidae

Genus: Dreissena

Species: polymorpha

Binomial nomenclature: Dreissena polymorpha (Pallas, 1771)

Synonyms: Mytilus hagenii, Tichogonia chemnitizii, Mytilus polymorphous

Common Names: Wandering mussel, Zebra mussel

• Species Identification

            Dreissena polymorpha can be identified through its shell which is a triangular or trigonal shell with sharply pointed hinge ends. These ends are usually referred to as umbos.4 The height of the shell makes up approximately 40-60% of the total length of the shell.4 Overall, the maximum size of the species is between 3 and 5cm. Most characteristic of D. polymorpha is the prominent banding pattern of dark and light shades on the shell. The name ‘polymorpha’ is descriptive of these shades since the species has several variations in the color, the pattern and the shape of the shell depending on the type of substrate used in making of the shell, the density and the aggregation. The common name ‘zebra’ is also descriptive of the brownish yellowish or greenish background color with clearly defined dark and light zigzag color shades of the shell

1. Distribution

            Several researchers have demonstrated that Dreissena polymorpha is native to the Black Sea, Caspian and Aral Sea basins.4 However, archaeological evidence of their presence in Western and Central Europe has also been found, hence their reappearance in Northern American waters, the Great Lakes basin and the European water mass has been viewed as a return migration that was necessitated via shipping canals and interconnecting water ways. It is therefore regarded as an alien species because it has invaded new habitats. The history of alien distribution can be traced to the 18^th and 19^th centuries when the distribution started expanding from the traditional native ranges to new habitats.

1. Ecology and Impacts of Dreissena polymorpha Colonization

            Dreissena polymorpha colonizes rivers, lakes and brackish lagoons. They prefer habitats such as the calm waters upstream of lakes or dams. They utilize hard surfaces such as rocks and macrophytes as substrates of attachment. D. polymorpha species spawns between the months of May and July. The fertilized eggs develop into veligar larvae that live planktonically for two to four weeks. Veligar larvae utilize its velum to swim rapidly as its development progresses to a final larval stage whence it begins to attack on hard surfaces. The distribution of this species in its habitat greatly varies because the larvae are capable of swimming and may therefore invariably drift to downstream levels during its pelagic phase of development.

            Additionally, the high reproductive output of D. polymorpha and their capacity to extend the larval planktonic development stage enables the species to disperse rapidly. Both the larvae and the adult D. polymorpha spread to new habitats as fouling on water vessels or fishing equipment. They can also be distributed in ballast water. Generally, D. polymorpha out competes native species. Their mass occurrence slows down the process of eutrophication thereby indirectly promoting blue green algal bloom. Increasing biodeposition ameliorate the ambient conditions necessary for benthic macro vegetation as well as directly promoting an increase in benthic deposit feeders4. Moreover, Zebra mussels are voracious filter feeders. This feeding mechanism increases the water transparency. The pseudofeces excreted by the organisms together with non food particles are deposited on the floors of the lakes where they act as principal substrates to benthic organisms. Decreased eutrophication and algal bloom is a direct threat to some aquatic species such as fish. As for native species of the genera Anadonta and Unio, D. polymorpha uses them as a hard substrate for attachment effectively starving them since they can no longer engage in undisturbed filter feeding.

            Several phylogenetic studies have also demonstrated that there is a possibility of hybridization between the morphologically similar D. bugensis and D. polymorpha. Laboratory techniques have succeeded in creating a hybrid between the two species by pooling their gametes. Even though inter specific fertilization is a rare phenomena in natural circumstances, it is nonetheless feasible.

            Among all the invasive vertebrates in our water systems, the bivalve D. polymorpha has continued to inspire phylogenetic studies eliciting a preponderance of papers simply because unlike all other species whose spread to alien habitats is but a man cause, this bivalve species has strained the economy and esthetically of the beaches and waterways as well as directly affecting aquatic life.5 After its introduction into the Great Britain in 1824, it transcended into the Great Lakes basin through the Great Atlantic waterways and into the North America. This paper succinctly analyzes such phylogenetic studies with a view to presenting the notable works on D. polymorpha.

PHYLOGENETICS

1. Taxonomy and Systematics

            Unlike other invasive species that are mainly ignored, two mollusk biofouling bivalve species of the genus Dreissena have not escaped attention due to their economically disastrous colonization of alien habitats. Dreissenids have been classified and reclassified several times at all classification levels. These reclassification attempts have brewed considerable confusion as regards phylogenetic relationships at the family level6. Such confusion is currently present in Russia where biologists and systematics have been utterly unable to reconcile the many reclassifications into a uniform genus Dreissena taxonomic history. Initially D. polymorpha and D. bugensis were classified as one species mainly due to morphological identity before D. bugensis was reclassified as a sub species of D. rostriformis and later on accepted as a distinctly different species.

            Some systematics argue that the genuses Mytilosis and Dreissena evolved from Congeria extinct branches7 While others typically maintain that Congeria and Dreissena evolved from Mytilopsis; then considered as subgenus of Congeria according to Russian taxonomists.7 Such is the confusion and complexity in the classification of dreissenids. Typically accepted as Ponto-Caspian natives, the invasion history spans from human activities in aquatic ecosystems has transcended into alien habitats globally. Their distribution is the cause for a preponderance of papers since they are bad news as they stifle indigenous aquatic species and severe environmental alterations due to their attachment mechanisms and biofouling. However, in some habitats they are deemed beneficial because Zebra mussels are voracious filter feeders. This feeding mechanism increases the water transparency. The pseudofeces excreted by the organisms together with non food particles are deposited on the floors of the lakes where they act as principal substrates to benthic organisms.

Gregory et al (2006) posits that the evolution of these two species occurred in the Ponto-Caspian Sea basin and that this evolution was marked by dynamic instability that occurred over multiple timescale. He additionally reiterates that evolution occurred in a unique environment that predisposed these species to invasiveness.8

            The endemic ranges of Zebra mussels are found in the brackish and freshwaters in the Northern regions of the Ponto-Caspian. Dreissena polymorpha began expanding its distribution beyond its endemic range in the eighteenth century following the construction of water ways and canal systems in the Eurasian water basin.9 Within North America it is believed that the invasive specie was first reported in 1988 in Lake St. Clair, Ontario. This species was believed to have been introduced into the lake as pelagic veligar larvae in the transoceanic ballast waters.10 This marked the beginning of an expansive colonization of the vast regions of interconnecting waterways that connect Europe and North America.

            Despite the presence of substantial historical information about the origins of the invasive species; Dreissena polymorpha, ultimate sources remain elusive and subject to a lot of speculations and hypotheses. The Ponto-Caspian basin is however regarded as the most plausible origin of the species and it is from this endemic range that the distribution and colonization spread top the Great Lakes basin in the United States of America since the 1980s11

            A second dreissenid, the Dreissena bugensis; the Quagga mussel, a native to the Northern Black Sea is currently displacing the Zebra mussel from the Great Lakes. In a study that was carried out to provide an insight into the invasion history and document the patterns of genetic diversity12 that exist between the endemic invasive populations, it was established that the Ponto-Caspian basin is the origin of the invasive species. Their distribution was largely consistent with the molecular ecological data from the invasive mollusks from the Black sea drainage. This suggested that colonization was as a result of a single founding population.

            Increasing global distribution heightened human aquatic activities calls for discerning these morphologically similar dreissenid species; Dreissena polymorpha, Dreissena bugensis, Dreissena rostroformis, Dreissena stavkovici and M. leucophaeta. These species also share several life history characteristics such as the pelagic veligar swimming larval stage and the byssal threads used for attachment onto hard surfaces. Older marker systems such as the mitochondrial gene loci markers are considerably best suited in the analysis of the taxonomic classifications since they are comparatively strongly differentiating markers. Often they are able to yield pertinent information in invasion biology even in cases of relatively short evolutionary periods. These markers form monophyletic groups because of their efficiency in small population sizes. The result is a rapid lineage sorting. However, these methods are still not encompassed in an international regulatory umbrella of universal scientific rules.

            Generally, biological invasions are very difficult to notice especially when the invading species is morphologically similar to the native species or when their effects on the aquatic ecosystem are initially not very different from that of past invaders.3 More cryptic invasions usually result when two or more of a morphologically similar species complex effectively colonizes a new habitat inn rapid succession. In most cases it is only genetic studies that have the capacity to provide evidence on biological invasion as was in the case of Dreissena polymorpha when its colonization was succeeded with a morphologically similar non native dreissenid species; the Quagga mussel: Dreissena begensis. For a long time, both the aggressive aquatic invaders, Quagga and Zebra mussel were inadvertently referred to as a single species.

            In a study carried out by Therriault et al(2004), mitochondrial gene sequencing was used to determine the phylogenetic relationships that exit between the three genuses; Dreissena, Congeria and Mytilopsis. The use of molecular methods to determine the relationship was mainly driven by the need to reconcile the highly discordant taxonomic data that have existed for a very long time.6 Through the use of mitochondrial DNA restriction and nuclear digests it was possible to identify species based on their distinct molecular characteristics as opposed to the unreliable morphological characteristics. The results of I6S  D. rostriformis and Dreissena bugensis fragments analyzed on the NJ tree denoted that these two species differed by one nucleotide. Moreover bootstrap support at the distinct nodes was weak clearly pointing at the possibility of a common ancestry. Additional analysis using maximum parsimony and NJ tree maintained the monophyletic nature of each taxon. Intra specific differences were further analyzed between the dreissenids. Restriction digests of both the mitochondrial genes and the nuclear genes yielded characteristic banding patterns that were consistent with other phylogenetic analyses,6 As shown below;

The figure above shows the phylogeny of the genus Dreissena computed using sequences (606 bp) of the mitochondrial COI gene. In the figure the distinct haplotypes for the Dreissena polymorpha complex are denoted at branch tips. For Dreissena caputlacus and Dreissena stankovici sampling locations are denoted and they correspond to the haplotype networks presentations figure. The phylogeny is constructed by using the Bayesian approach. (Illustration adapted from Gelembiuk et al p. 1038)

            Reporting on the phylogeography of both the endemic and invading Dreissena polymorpha populations, Gemma et al (2005) demonstrated that there exists a wide diversity in the haplotypes of mitochondrial cytochrome oxidase genes(COI haplotypes A-J). Only the haplotypes A and B were predominant in the invaded regions. These two haplotypes are also resident in the Black Sea drainage, River Volga and other regions closer to the Volga delta as well as in every invasive population in the Eurasia and the Northern American waters. The significance of this finding is that it presents the Black Sea drainage as the origin of Dreissena polymorpha; a finding that is compatible with other works. However the Caspian Sea contained the haplotypes of C, D and D2 as well as the haplotypes A and B. With respect to intraspecific genetic polymorphism among  members in the genera Dreissena; D. polymorpha, D. rostriformis and D. bugensis had lower genetic diversity.

            Using statistical parsimony networks, the haplotype relatedness for all the dresseinids; Dreissena polymorpha, Dreissena caputlacus and D. stankovici were analyzed for intraspecific genetic polymorphism.8 For Dreissena polymorpha complex there were two apparent clusters centered on haplotype B and haplotype F. Central in the D. polymorpha cluster is the haplotype F which was five mutational steps further from haplotype B and haplotype E. Haplotype E and B were two steps further apart. Coalescent simulations were performed under unique event mutation polymorphism to produce a 76% probability hence intuiting that haplotype B was the ancestral haplotype for Dreissena polymorpha. Additionally another haplotype Ω₁ that exist between the haplotypes of B and E showed the highest probability of being the ancestral haplotype for the whole D. polymorpha complex. For D. caputlacus, Ω₂; the inferred haplotype exhibited a root probability of 39%_..8 Analysis of haplotype networks yielded an evidence of historical population expansion exhibited in star like phylogenies. This patten is in consistence with patterns of population growth.

(Adapted from Gelembiuk et al. p. 1037). The figure shows the haplotype networks for the three Dreissena species. In the figure point mutations are denoted by dashes(—)The are circled are representative of haplotype frequencies from samples collected from;

A;  Dreissena polymorpha complex from endemic range. The most probable ancestral root is represented by  Ω1.B;  Dreissena caputlacus from Seyhan in Turkey. The most probable ancestral root is represented by   Ω2. C;  Dreissena stankovici from Lake Ohrid and Prespa.  The most probable ancestral root is represented by Ω3

            It has been further demonstrated that genetic imprints in an ecological habitat that is prone to disturbances justified the existence of an evolutionary history that produced very distinctive genetic patterns with specific neutral markers that predisposed the entire population to invasiveness. If the old world species and the new world species are compared it can be determined that the old world species have a history of being subjected to greater anthropogenic and natural disturbances hence invasiveness. There are data that support this directionality of invasion.8 Because the Ponto-Caspian basin is home to numerous natural and anthropogenic disturbances marked by fluctuations of environmental factors that act as growth determinants, the development of invasiveness by D. polymorpha can be traced to these habitats.

            It is upon this basis that Gelembiuk et al(2006), sought to determine the genetic signals of  D. polymorpha population bottlenecks and its increased expansion to alien habitats, haplotype diversity and the development of genetically distinct population sets in retracing the evolutionary history of Dreissena from its original habitat; the Ponto-Caspian region. The Ponto-Caspian basin is a region that has been characterized by dynamic and large scale instability on a couple of timescales in the history of the evolution of the world. Such anthropogenic and natural environmental upheavals are creating a unique evolutionary environment  that even perhaps predisposes the inhabiting species to invasiveness. Even though the Ponto- Caspian basin is depauperate in species, it nonetheless contains certain peculiar species with unique characteristics and the Dreissena species constitutes a comparatively large portion of these sets of species flocks. Studies have been carried on taxonomy and systematics of Dreissena but there still exists a paucity of data on the effect of specific climatic and geological events  in determining the genetic signatures and intra specific genetic patterns  of Dreissena populations.

             Like many other genus that constituted the Ponto Caspian fauna, the genus Dreissena can be traced to have originated from the ancient Tethys Sea and subsequently the brackish Paratethys Sea. Historical natural upheavals such as the extinction of the Cretaceous-Tertiary and subsequent waterways formations prompted the basal divergence such as that of the Dreissena sensu stricto and Pontodreissena during the Palaeogene event. Estimates of genetic imprints from the basal divergence from the ancient relict populations are in consistence with imprints the lakes in the Balkans. Analysis therefore points to the fact that the high haplotype diversity and their relative abundance confirm the ancestral source of Dreissena genus colonization and expansion. The closest taxa to the dreissenids in the Balkan lakes are the Ochridospongia rotunda resident in Lake Baikal.8

1. Molecular and Genetic Studies of Dreissena polymorpha

            Since the introduction of Dreissena polymorpha into the Great Lakes, noteworthy efforts have been channeled towards the examination of economic and ecological implications of this species to the ecosystem and to the economic prosperity inn the Great Lakes. These studies have ideally been driven by the fact that genetic studies are fruitful in research about the origin of species, dispersal rates and genetic mediated developmental processes. However, genetic studies that mainly concentrate in small localities with identical  ambient ecological conditions have proved to be limited in scope and benefits with respect to research.10 Geographic gene frequency examinations are instrumental in yielding information about the variations in genetic makeup hence providing a benchmark for the assessment of the origins of species and the gene flow rates. To assess the linkage between genetic variations of species in a population and the growth of such species multiple polymorphism studies carried out amongst individuals in a cohort.13

            Genetic studies have demonstrated that in certain circumstances substantial genetic shifts can be identified in the population. For instance, allelic frequencies in a population shift significantly. Loss of alleles during the development of novel populations leads to a substantial reduction in heterozygosity.14 Reduction in heterozygosity has great influence on the evolutionary and life historical aspects of any species. This has been demonstrated to considerably affect the survivorship and consequently the growth rate among marine inhabiting mollusks. Zebra mussel has many polyphormic loci in addition to the existing allelic variants among populations isolated in North American waters. This specifically points to the fact that earlier on in the history of the distribution of Dreissena polymorpha colonists might have been released into this ecosystem where they gradually and individually established their own ecological niche. 13

            To ascertain the existence of genetic variation and assess the allozyme frequencies and the heterozygosities resulting from the loss of alleles, a study was carried out in the Great Lakes and in European colonies to yield a reliable data detailing the effects of colonization   among these populations as well as their dispersal rates. In addition the cohort study aimed at establishment the linkage that exists between the growth rates and the genetic variations inherent in the Dreissena polymorpha populations. In 1991, specimen of zebra mussels collected from 7 designated locations in North America were enlisted in the study.

Using allozyme electrophoresis in cellulose acetate gels, the study demonstrated the variation in allele frequencies all through the survey . Out of the thirty six alleles were taken through the study eleven loci were examined, two infrequent alleles(LDH³ and IDH³) were specifically unique to the populations derived from Europe while three(PGI⁵, PP³ and MPI) infrequent alleles were specific to the Great Lakes Basin Dreissena polymorpha populations. The allele frequency differences between the two continents was also established with allele MDH² dominating the Great lakes whileMDH¹being comparatively dominant in Europe. However, the variation in frequencies was also noticeable between the Great Lakes populations. For instance, Lake Erie and Lake St. Clair exhibited significant heterogeneity at 50% of the loci that were studied. Additionally, IDH And MDF variations between Lake Erie and Lake St. Clair showed marked variation from near fixation to an almost equal level among populations collected from nearby localities.13

            Populations collected from within the Great Lakes showed a variation in the heterozygosity but the founder populations collected from the Lake St. Clair  and the European populations did not significantly differ(F_[1,10]=3.196). The Oneida lake population possessed a significantly lower mean heterozygosity as compared to those of Lake St. Clair(F_[1,10]=5.34).13 This is attributable to the lack of variation between the MDH and LDH loci.

The patterns of heterozygosity and gene frequency testify to earlier explanations that sought to define the origins of the species. Variations in the Great Lakes Basin also attest to the unexpected behavior of individual invading species. The presence of unique allelic variants suggest that even the Great Lakes populations must also have a source of their populations. The gene frequency variations point to the fact that distribution of the species is localized.

            Although the gene frequency shifts in Dreissena polymorpha is relatively modest, a significant degree of gene differentiation exists among populations found in areas that were the first to be colonized. The rapid spread in the Great Lakes basin can be attributed to the pelagic veliger larvae and the high fecundity.15 However, neither pelagic veliger larvae nor high fecundity enhances  genetic discontinuity hence the genetic variations and the reduction in homogeneities deserve closer scrutiny to determine the real factors at play in genetic discontinuity.

            Gene frequency shifts in Dreissena polymorpha occur due to selective differences among species in a locality or the compounded effects of sampling founders. Both processes however, require restricted dispersal mechanisms. Gene frequency shifts can result from selective differences if gene flow is curtailed. This means that a minimal number of colonists can produce gene frequency shifts at each locality when the dispersal distance is reduced during the process of colonization. This explanation is largely accepted as opposed to the pelagic veliger larvae dispersal since this process is largely restricted during early stages of formation. Moreover, if the number of founder colonies is small, then the variation in the heterozygosities should be much lower if compared to other localities. The low heterozygosities in Oneida Lake individuals is a testament to this fact.

            Marine bivalve mollusks have considerable deficiencies of heterozygous genotypes mainly due to random mating between members of the population. However, heterozygous individuals isolated in a specific cohort often develop larger shell sizes in comparison to individuals isolated from a homozygous group. This can be explained by the fact that combined effects exhibited by heterozygous enable them to have a superior growth rate but little settlement success. However, these growth deficits are not genetically determinable as there is no scientific consistent evidence supporting the presence of heterozygous deficits at polymorphic loci among the Dreissena polymorpha populations. Additionally, there is no concrete and consistent evidence that has established the relationship between the degree of heterozygosity and the rate of growth or early presettlement growth. However, Garton et al(1991) established that there was a relationship between individual heterozygosity and the size. This evidence suggested that in heterozygous individuals, the growth and rate of survival was much greater.17 Analysis of evidence from genetic studies therefore suggest that genetic variability has no relationship with dispersal success.

            While some experts agree that genetic diversity plays a central role in successful invasion by non indigenous invasion some disagree. Investigations have centered in enlisting and confirming the founding sources and the genetic variability thereof. In study carried out by C.A Stepien et al(2002), the researchers analyzed the population genetic structure, genetic diversity and divergence patterns. A total of 280 samples of Dreissena polymorpha were drawn from the sample population in addition to sixty three putative rapidly amplified polymorphic DNA(RAPDs) gene loci. Representative samples of Dreissena bugensis from both the Eurasian and North American sampling sites were enlisted in the study. Results demonstrated that the exotic populations had greater genetic variability similar to the species population in Eurasian populations.

This pointed to the fact that there was a predominantly high number of founding individuals hence supporting the concept of multiple colonizations.18  It is possible to detect any prior changes in species population through the comparison of allele numbers at a specified locus and the results compared with the heterozygosities in the whole populations. Because the latter decreases at a relatively slower rate that the former after a bottleneck, however the technique has not been largely eventful due to the complexity of choosing the right marker. With the development of the novel micro satellite markers, a large number of alleles in a micro satellite marker allows for a more eventful statistical computation of heterozygosity at a locus when hybridization is carried out at mutation drift equilibrium.

            In a study carried out by Naish et al(2001), the researchers used the concept of high mutation rate that usually occurs at the micro satellite loci as a basis of obtaining  demographic information about the founder events of the biological invasion, colonization and expansion of D. polymorpha after its initial introduction into the alien habitats.19 The researchers isolated a total of 5 trinucleotide micro satellite loci from a partial cDNA library. The allelic diversity in all the isolated loci was very high. Using such information statistical computation of heterozygosity is possible. Isolation of micro satellite markers to explain the life history traits like planktonic veligar phase of larval development, byssal thread attachment and even predict future endemic and non endemic expansion ranges.

            Several other studies based on allozyme electrophoresis have indicated that Dreissena polymorpha has extremely high genetic variability. In fact they rank among the highest values of heterozygosity reported among animals. Marsden et al(1996) reported heterozygosity values of between 29% to 46% in the North American populations and 27% to 46% among European populations.20 In a study carried out by Müller et al(2001), heterozygosities ranged from 29% to 44%. These values include even the monomorphic locus IDH. Apart from the other factors such as the high pelagic larval dispersal ability, the planktonic larval phase and the  rapid long distance dispersal by water vessels, the high level of genetic diversity can also be explained by the fitness differences that exist between genotypes.3

The morphologically similar veligar larvae of the Quagga mussels and the Zebra mussels can also be differentiated using a novel technique using specific dresseinid PCR primers developed from bivalve DNA sequences. These primers amplify a specific region of the cytochrome c oxidase subunit I mitochondrial(COI) gene when the dressenid mtDNA is analyzed, the two species can be differentiated based on the differences in mtDNA sequences which are definitive for either the quagga or zebra.21 Prior studies by Claxton and associate researchers had determined that the adult zebra or quagga and their associated post larval stages could be differentiated using RFLPs (restriction fragment length polymorphisms) of the mitochondrial COI gene. Using the restriction enzyme NlaIV RFLP technique comfortably analyzes and distinguishes the two bivalve mollusca species. Moreover diagnostic differences in adult Dresseina polymorpha  and its potentially co-occurring bivalve: the Dreisseina bugensis can also be determined when the 710-bp nucleotide sequence fragment of the mitochondrial COI gene is sequenced.22 The differences in the fragment numbers and the sizes of nucleotide sequences can also be used to distinguish the morphologically identical larvae of the two bivalves.

            The spread of D. polymorpha has been rapid with disastrous ecological and economic consequences. Its spread has generated both disjunct and contiguous populations. From new colonies acting as the founders new populations with markedly different phenotypic and genetic signatures have arisen from the alien geographic locations. Populations from the Great Lakes and Europe were analyzed for genetic differentiation  using starch gel electrophoresis. Results confirmed earlier findings on heterozygosity for instance for all the samples taken though genetic differentiation tests, heterozygosities ranged from 27% to 47%. However, populations derived from the European distribution had a slightly lower heterozygosity range for instance 27% to 35% when compared to the North American populations with a heterozygosity range 30% to 43%, with a P< 0.05 when analyzed using the Mann Whitney rank test. The genetic distance coefficient is much higher among the European populations as compared to the North American populations.23 The genetic distance coefficient range among the European populations was 0.007 to 0.139 while the latter was between 0.00-5 and 0.025. these differences between the populations can be explained by the difference in allellic frequencies at a certain number of loci as opposed to the absence or presence of a certain rare allele occurring at a single locus. The two populations also clustered separately with a genetic distance coefficient of 0.058.23

            When the genetic variability was analyzed by F distribution test, the results attested to the fact that there was considerably less genetic differentiation among the North American populations in comparison to the European populations. For instance in North American population; F_ST= 0.019 while in Europe F_ST=0.079.23 All these data therefore confirm that D. polymorpha populations are not very differentiated. This can be explained by the fact that these geographical areas are closely interconnected and there is a possibility of mixing up of water flow and therefore species and genetic interchange. Populations that are widely geographically isolated have lesser chances of mixing together hence the considerably high genetic differentiation values. 

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Mueller, C. Jacob & Griebeller M. E. Genetics on Invasive Species. In Leppäkoski, Erkki.,          Stephan Gollasch, Sergej Olenin. Invasive Aquatic Species of Europe: Distribution,    Impacts, and Management, 2002. p. 173

Müller, J., Wöll, S.,  Fuchs, U., & Seitz, A. Genetic interchange of Dreissena polymorpha           populations across a canal. Heredity, 2001, 86, 103–109

Nalepa, T. F & Schloesser, W. Donald. Zebra Mussels. Biology, Impacts and Control. 1993. p.    227-233

Naish, A. Kerry & Boulding, G. Elizabeth. Trinucleotide microsatellite loci for the  Zebra           Mussel; Dreissena polymorpha, an invasive species in Europe and North America.     Molecular Ecology Notes, 2001, 1, 286–288

Pimental, D.,  Lach R., Morrison D. Environmental and Economic Costs of Non Indigenous       Species in the United States. Bioscience, 2000, 50, 53-65.

Starobogatov Y.I, Andreeva, S. I. Taxonomy and paleontology. In: Fresh water Zebra mussel                 Dreissena polymorpha(Pall.) (Bivalvia, Dreissenidae): Systematics , Ecology,       Practical Meaning. Russian Academy of Sciences. Nauka Press Moscow. 1994. p.             18-46

Therriault, T. W., Orlova, M. I., Docker, M. F, Heath, D. D., and Maclsaac, H. J. Molecular        Resolution of the Family Dreissenidae(Mullusca: Bivalvia) with emphasis on Ponto- Caspian Species, including first report of Mytilopsis leucophaeata in the Black Sea           Basin. Molecular Phylogenetics and Evolution. 30: 479-489

Ricciardi A., Maclsaac H. J., Recent Mass Invasion of the North American Great Lakes by the Ponto-Caspian species. Trends in Ecology and Evolution. 2000.15, 62-65

1    Müller, J., Wöll, S.,  Fuchs, U., & Seitz, A. Genetic interchange of Dreissena polymorpha populations across a canal. Heredity, 2001, 86, 103–109

2    Pimental, D.,  Lach R., Morrison D. Environmental and Economic Costs of Non Indigenous Species in the United States. Bioscience, 2000, 50, 53-65.

3    Mueller, C. Jacob & Griebeller M. E. Genetics on Invasive Species. In Leppäkoski, Erkki., Stephan Gollasch, Sergej Olenin. Invasive Aquatic Species of Europe: Distribution, Impacts, and Management, 2002. p. 173

3    Ibid., p. 176

4    Bimbaum, Christina. NOBANIS-Invasive Alien Species Fact Sheet-Dreissena polymorpha. www.nobanis.org. p. 2

4    Ibid., 3

4    Ibid., p. 2-3

4    Ibid., p. 4

5    Morton, Brian. The Aquatic Nuisance Species Problem: A global Perspective and Review. p. 1-10

6    Therriault, T. W., Orlova, M. I., Docker, M. F, Heath, D. D., and Maclsaac, H. J. Molecular Resolution of the Family Dreissenidae(Mullusca: Bivalvia) with emphasis on Ponto-Caspian Species, including first report of Mytilopsis leucophaeata in the Black Sea Basin. Molecular Phylogenetics and Evolution. 30: 479-489

7    Starobogatov Y.I, Andreeva, S. I. Taxonomy and paleontology. In: Fresh water Zebra mussel : Dreissena polymorpha(Pall.) (Bivalvia, Dreissenidae): Systematics , Ecology, Practical Meaning. Russian Academy of Sciences. Nauka Press Moscow. 1994. p. 18-46

7    Ibid., p. 31

8    Gelembiuk, G.W., Gemma, E.E and Eunmin Lee. Phylogeography and Systematics of Zebra Mussels and Related Species. Molecular Ecology, 2006, 15, 1033-1050.

9    Bij de Vaate, A., Jazdzewski K., Ketelaars H. A. M., Gollasch S, Van der Veld E. G. Geographical patterns in range extension of Ponto-Caspian macroinvertebrate species in Europe. Canadian Journal of Fisheries and Aquatic Sciences, 2002, 59, 1159–1174.

1             0             Herbert, P. D. N., Muncaster B. W., Markie G.L. Ecological and Genetic Studies on Dreissena polymorpha                 (Pallas): A New Mollusc in the Great Lakes. Canadian Journal of Fisheries and Aquatic Sciences, 1989. 60,             740-756

1             1             Lee C.E. & Bell M.A. Causes and Consequences of Recent Fresh Water Invasions by Salt Water                 Animals.                 Trends in Ecology and Evolution, 1999. 14, 284-288.

1             2             Gemma, E. M., Gregory, W. G,. Vadin, E. P., Marina, I.O., Lee, E. C. Molecular Ecology of Zebra Mussel Invasions. Molecular Ecology ,2005, 15,1021-1031.

3    Ibid., p. 176

6    Ibid., p. 482

6    Ibid., p.485

8    Ibid., p. 1037

8    Ibid., p. 1038

8    Ibid., p. 1042

8    Ibid., p. 1042

1             0   Ibid., p. 746

1             3             Nalepa, T. F & Schloesser, W. Donald. Zebra Mussels. Biology, Impacts and Control. 1993. p. 227-233

1             4             Boileau, M. G. & P. D. N. Herbert. Genetic Consequences of Passive Dispersal in Pond Dwelling               Copepods. Evolution. 1990, 24;721-733.

1             3             Ibid., p. 228

1             3             Ibid., p. 230

1             3             Ibid., p. 231

1             5             Ricciardi A., Maclsaac H. J., Recent Mass Invasion of the North American Great Lakes by the Ponto-        Caspian species. Trends in Ecology and Evolution. 2000.15, 62-65

1             7             Cristescu M. E. A., Herbert P. D. N., Grigorovich I. A., Maclsaac H.J. An invasion history for Cercopagis                   pengoi based on mitochondrial gene sequences. Limnology and Oceanography, 2001.46, 224-229

1             8             C. A. Stepien , C. D. Taylor & K. A. Dabrowska. Genetic Variability and phylogeographical patterns of a                non indigenous species invasion: a comparison of exotic vs. native zebra and quagga mussel populations.    Journal of Evolutionary Biology, 15, 2,p, 314-328

1             9             Naish, A. Kerry & Boulding, G. Elizabeth. Trinucleotide microsatellite loci for the  Zebra Mussel;              Dreissena polymorpha, an invasive species in Europe and North America. Molecular Ecology Notes, 2001,              1, 286–288

2             0             Boileau, M. G. & P. D. N. Herbert. Genetics of the Zebra Mussel (Dreissena polymorpha) in Populations from the Great Lakes Region and Europe. In Zebra mussels : biology, Impacts, and control. Edited by Nalepa and Schloesser. Lewis Publishers, Boca Raton.1993. p.227-238.

3    Ibid., p. 178

2             1             Claxton, W. Trevor and Boulding, Elizabeth G. A new molecular technique for identifying field

                collections of zebra mussel (Dreissena polymorpha) and quagga mussel (Dreissena bugensis) veliger larvae         applied to eastern Lake Erie, Lake Ontario, and Lake Simcoe. Can. J. Zool., 1998, 76: 194–198.

2             2             Baldwin, B. S.,  M. Black, O. Sanjur, R. Gustafson, R. A. Lutz, and R. C. Vrijenhoek. A   Diagnostic           Molecular Marker for Zebra mussels (Dreissena polymorpha) and Potentially co-occurring bivalves:          Mitochondrial COI. Molecular Marine Biology and Biotechnology, 1996, 5(1). 9-14

2             3             Marsden, J.E., Spidle, A., May, B. Genetic Similarity among Zebra Mussel populations within North         America and Europe. Canadian Journal of Fisheries and Aquatic Sciences, 1995. 52, 836–847.

2             3             Ibid., p. 839

2             3             Ibid., p. 840