Athanasios D. Baxevanis

Is polyploidy a persevering accident or an adaptive evolutionary pattern? The case of the brine shrimp Artemia

Asexual organisms are confronted with substantial drawbacks, both immediate and delayed, threatening their evolutionary persistence. Yet, genetic associations with asexuality may refresh the gene pool promoting adaptation of clonal lineages; polyploidy is one of them. Parthenogenesis itself and/or polyploidy are responsible for the maintenance and spread of clones in Artemia, a sexual-asexual genus of halophilic anostracans. We applied flow cytometry, microsatellite genotyping, and mtDNA sequencing to 23 asexual populations. Artemia parthenogens have evolved multiple times either through hybridization or spontaneously. Nine out of 23 populations contained clones of mixed ploidy (2n, 3n, 4n). Most clones were diploid (20/31) while two and nine clones were triploid and tetraploid, respectively. Apomictic triploids and tetraploids formed two distinct groups of low genetic diversity compared with the more divergent automictic diploids. Polyploidy is also polyphyletic in Artemia, with triploids and tetraploids having independent origins from different sexual ancestors. We discern a pattern of geographical parthenogenesis with all clonal groups being more widespread than their closest sexuals. In favour of a specialist model, asexual diploids are restricted to single locations and are strikingly segregated from generalist triploids and tetraploids occupying a variety of sites. This is a rare pattern of mixed life-history strategies within an asexual complex.

Salinity effects on maturation, reproductive and life span characteristics of four Egyptian Artemia populations (International Study on Artemia. LXVIII)

Three parthenogenetic Artemia populations, i.e. two coastal from Borg El-Arab and El-Max saltworks and one from the inland Qarun Lake, and a bisexual strain (Artemia salina) from the inland carbonate lake of Wadi El-Natrun, all from upper Egypt were assayed for their response in 5 salinities (i.e. 35, 80, 120, 150 and 200 g l−1). The experimental procedure was carried out under laboratory conditions, where the effects of salinity on maturation and ten reproductive and life span characteristics were investigated. The parthenogenetic Egyptian populations are more euryhaline compared to the bisexual one. The two coastal parthenogenetic populations appeared to be very similar in maturation rate and reproductive output at all salinities tested. The inland asexual strain showed a different reproductive response to the elevation of salinity from the two coastal populations. Discriminant function analysis has proven to be a useful tool in determining the differential response of closely related Artemia populations. The bisexual population showed significantly lower reproductive output compared to the parthenogenetic ones and performed best at 35 g l−1; this is the first record of an A. salina population inhabiting a carbonate lake. These findings may provide valuable information on Artemia biodiversity in an area where very little is known. %

Salinity effects on survival, growth and morphometry of four Egyptian Artemia populations (International Study on Artemia. LXVII)

Four Artemia populations from northern Egypt, a bisexual one from Wadi El-Natrun Lake, two coastal parthenogenetic ones from Borg El-Arab and El-Max saltworks and an inland parthenogenetic form from Qarun Lake, were assayed for their survival, growth and morphometric responses measured in laboratory experiments at salinities of 35, 80, 120, 150 and 200 g l−1. The survival rate was determined using regression analysis and analysis of covariance. The bisexual population (Wadi El-Natrun: WN) exhibited its best survival at 80 g l−1. All parthenogenetic strains studied performed similarly (in terms of survival) at all salinities investigated. The population growth rates were based on Von Bertalannfy’s equation. In all salinities, WN population had the lowest growth rate (based on K values) among all Artemia populations tested. It is obvious that parthenogenetic populations tolerate a broader range of salinities compared to the bisexual one. The two coastal asexual strains had similar survival, growth and morphometric characters at all salinities. Discriminant function analysis based on specific morphometric parameters permitted the assignment of adult Artemia individuals to their population of origin with a score as high as 84.6%. Furthermore, specific morphometric parameters (such as the furcal length and the number of setae in each furcal branch) are suitable discriminating characters among the populations studied.

Effects of salinity and temperature on reproductive and life span characteristics of clonal Artemia. (International Study on Artemia. LXVI)

An apomictic clone of the tetraploid parthenogenetic Artemia population from M. Embolon (Thessaloniki, Greece) was assayed for 10 reproductive and life span characteristics under laboratory conditions (in various salinity and temperature regimes). Salinity was proved to have significant impact on the majority of the characters used in this study. Discriminant function analysis gave an overall prediction of 97.32% over the three salinities (50, 80 and 120 ppt). The temperature of 30 °C seemed to be an extreme one affecting significantly nearly all of the studied variables. The overall prediction according to the discriminant analysis was 94.69% among the three temperatures (22, 26 and 30 °C). The clone performed best at 80 ppt and 22 °C. The data presented in this study may generate useful suggestions to investigate the potentiality of using a single genetic lineage in order to visualize the effects of different environmental cues on a specific clone.

Inter- and intra-specific diversity of Pseudo-nitzschia (Bacillariophyceae) in the northeastern Mediterranean

A total of 92 cultured Pseudo-nitzschia strains, established between March 2007 and April 2010 from Greek coastal waters, were identified by using morphological and molecular (ITS2 region) characters. Twelve species were identified, among which P. brasiliana, P. subpacifica and P. hasleana were detected for the first time near Greek and East Mediterranean coasts. One P. delicatissima-like morphotype and another strain that closely resembled P. dolorosa were also found. Morphology and ITS2 phylogeny indicated that the level of diversity within the genus is higher than previously estimated (e.g. among P. delicatissima-like diatoms). Additionally, the ITS2 phylogeny revealed genetic variation within species, indicative of geographical differentiation (in P. brasiliana, P. fraudulenta, P. arenysensis, P. delicatissima and P. galaxiae). The majority of the cultured strains were also tested for domoic acid (DA) production. Strains of only three species, P. pseudodelicatissima, P. galaxiae and P. pungens var. pungens, were found capable of producing DA.

Peracarida populations of hard substrate assemblages in ports of the NW Aegean Sea (eastern Mediterranean)

This study deals with the structure of Peracarida populations in four ports in the NW Aegean Sea, Greece, and with the degree this structure is influenced by the particular biotic and abiotic conditions that prevail in the ports. Quantitative samples were taken during summer and winter in two successive years from artificial hard substrates and were analysed using common biocoenotic methods. The examination of approximately 81,250 specimens revealed the presence of 24 Peracarida species, the most dominant of which were Corophium acutum, Leptochelia savignyi and Elasmopus rapax. All species are very common and have been reported from many sites and assemblages in the N Aegean Sea. The ratios of certain Peracarida genera are discussed as possible indicators of environmental health that may be used in long-term biomonitoring programmes on the impact of pollution in harbours.

Porous genomes and species integrity in the branchiopod Artemia

Over the years, studies on interspecific hybridization have highlighted cases where gene exchange between taxa continues for a significant amount of time after speciation. The reasons for this lag of reproductive isolation relative to genetic isolation are largely unclear, and the question still remains whether the resulting hybrids represent novel biological (and taxonomic) diversity or merely an evolutionary liability. We provide strong indications in the branchiopod Artemia that hybrids between distantly related species may not be evolutionary inconsequential. Based on a global sampling of published and newly derived nuclear (ITS1) and mitochondrial (16S rRNA) sequence data from all representatives of the genus, we have identified natural hybrids between Artemia species (A. persimilisxA. franciscana, A. salinaxA. franciscana) separated by evolutionary interludes of tens of millions of years. Our combined analytical framework of cladistic and network methods provides evidence that hybridizations are the result of recent secondary contact following pronounced allopatric differentiation. The detection of mitochondrial introgression from A. persimilis to A. franciscana attests F(1) hybrid fertility. The reasons for this apparent unidirectionality of introgression are currently unknown but a likely explanation is provided based on morphometric divergence. We discuss the evolutionary implications of our results within the broader context of continental zooplankters.

Sharp Phylogeographic Breaks and Patterns of Genealogical Concordance in the Brine Shrimp Artemia franciscana

Genealogical concordance is a critical overlay of all phylogenetic analyses, irrespective of taxonomic level. To assess such patterns of congruence we have compiled and derived sequence data for two mitochondrial (16S rRNA, COI) and two nuclear (ITS1, p26) markers in 14 American populations of the hypersaline branchiopod Artemia franciscana. Cladistic analysis revealed three reciprocally monophyletic mitochondrial clades. For nuclear DNA, incomplete lineage sorting was evident presumably as a result of slower coalescence or male-mediated dispersal. Our findings capture the genealogical interval between gene splitting and population divergence. In this sense, strong indications are provided in favour of a superspecies status and ongoing speciation in A. franciscana.

Genetic characterization of common carp (Cyprinus carpio) populations from Greece using mitochondrial DNA sequences

Wild common carp from two lakes and two rivers in Greece were genetically characterized with sequencing analysis of two mitochondrial DNA segments: cytochrome b (1119 bp) and D-loop (646 bp). A total of 9 variable singleton sites and 7 unique haplotypes were detected. A common haplotype was found in three out of the four populations examined, which seems to be the ancestral one and represents the European origin of common carp from Greece. This haplotype could be also justified by the introductions reported with individuals belonging to the Central European race, into many natural habitats in Greece. Limited genetic variation — in Evros and Aliakmonas populations — could be due to bottleneck effects and small effective population sizes, whereas the different haplotypes found in Lake Volvi could represent different common carp stocks. Values of sequence divergence among Greek haplotypes ranged from 0.0006 to 0.0023. The Neighbour-Joining (NJ) phylogenetic tree constructed based on the combined sequences, reveals that the populations of common carp from Greece belong to the European group of populations — which is highly divergent from the South East-Asia cluster — and to the subspecies Cyprinus carpio carpio.

Genetic characterization of Argentinean Artemia species with different fatty acid profiles

The Anostracan genus Artemia is composed by several sibling species reproductively isolated, but identical or very similar in outward appearance. The genus shows also an underlying striking variability from the biochemical point of view, regarding especially the fatty acid profile of the cysts and nauplii. In Argentina, Artemia is represented by two bisexual species: A. franciscana and A. persimilis. Former studies have shown that A. franciscana is present in northern of 36º and that A. persimilis is constrained southwards of 37° S. In general, there is good agreement between morphological and cytogenetic comparisons of Argentinean populations with respect to species discrimination. However, new Argentinean Artemia populations are being analyzed morphologically and it becomes necessary to further investigate if the genetic adscription of these populations is congruent with the results obtained from the current morphological analyses. Restriction fragment length polymorphism (RFLP) analysis of a fragment of the 16S rRNA mitochondrial gene was used to investigate the genetic diversity and population structure of 10 new Artemia populations from Argentina. The mitochondrial DNA (mtDNA) results showed a similar pattern to that of previous cytogenetic and morphological analyses with the two Argentinean species appearing as highly divergent. The presence of A. persimilis in southern Argentina and the southernmost Chilean population was confirmed unveiling a novel picture of species distribution in the country. A. franciscana showed a unique haplotype. Populations of A. persimilis appeared highly structured, although their clustering did not follow a clear geographic pattern. The different Argentinean Artemia populations analyzed were characterized by high variability in their fatty acids, showing both marine- and freshwater-type profiles. For the first time, the investigation of the relatedness between the fatty acid composition in Artemia and genetic markers was attempted. The study aimed at the putative association of molecular markers with marine versus freshwater-type populations. A lack of correlation between RFLP patterns at mtDNA and the fatty acid (FA) profiles was found in the A. persimilis populations which was discussed from the point of view of two main genetic hypotheses and/or phenotypic plasticity.