Stefania Maniatsi

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.

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.

May parthenogenesis in Artemia be attributed to Wolbachia

Rickettsia-like maternally inherited bacteria are commonly associated with the induction of severe modifications of the reproductive behaviour of their hosts, such as parthenogenesis. Among them, the endosymbiont Wolbachia is the best known because it is widespread in many arthropods, including insects, terrestrial isopods and the exclusively aquatic ostracods. The genus Artemia (Crustacea, Anostraca) that consists of both bisexual species and parthenogenetic strains was—to our knowledge—for the first time tested for infection with Wolbachia. We screened 38 Artemia populations (parthenogenetic and bisexual) using a PCR assay. Our results clearly show that Wolbachia is not involved in the induction of parthenogenesis in Artemia.

The study of fkbp and ubiquitin reveals interesting aspects of Artemia stress history.

Research on stress responses in animals has increased greatly during the last decades. Though most studies focus on the cellular and molecular bases of the stress response mechanisms, the ecological and evolutionary aspects of stress responses gain more and more interest. Here, we use species and parthenogenetic strains of the genus Artemia, an extremophile model organism, to study, for the first time, a protein well known for its chaperone activity and its involvement in stress responses. More specifically, transcription and protein accumulation of an FK506-Binding Protein (FKBP) homologue were investigated under heat and salt stresses. Additionally, the mRNA levels of ubiquitin, a heat-inducible protein related to the proteasomal pathway, were quantitated under these conditions. Biochemical and phylogenetic analyses showed that the studied FKBP orthologue is a typical representative of the family that clusters with other crustacean sequences. The expression was increased in both fkbp and ubiquitin genes after salt and heat stresses. However, our results in combination with the fact that Artemia species and parthenogenetic strains, selected for this study, exhibit different heat or salt tolerance provide useful hints about the evolutionary significance of FKBP and ubiquitin. Regarding FKBP, mRNA expression and protein accumulation seem to depend on the environmental conditions and the evolutionary history of each Artemia population while ubiquitin has a clear and more conserved role under heat shock.