BackgroundGenetically divergent cryptic species are frequently detected by molecular methods. These discoveries are often a byproduct of molecular barcoding studies in which fragments of a selected marker are used for species identification. Highly divergent mitochondrial lineages and putative cryptic species are even detected in intensively studied animal taxa, such as the crustacean genus Daphnia. Recently, eleven such lineages, exhibiting genetic distances comparable to levels observed among well-defined species, were recorded in the D. longispina species complex, a group that contains several key taxa of freshwater ecosystems. We tested if three of those lineages represent indeed distinct species, by analyzing patterns of variation of ten nuclear microsatellite markers in six populations.ResultsWe observed a discordant pattern between mitochondrial and nuclear DNA, as all individuals carrying one of the divergent mitochondrial lineages grouped at the nuclear level with widespread, well-recognized species coexisting at the same localities (Daphnia galeata, D. longispina, and D. cucullata).ConclusionsA likely explanation for this pattern is the introgression of the mitochondrial genome of undescribed taxa into the common species, either in the distant past or after long-distance dispersal. The occurrence of highly divergent but rare mtDNA lineages in the gene pool of widespread species would suggest that hybridization and introgression in the D. longispina species complex is frequent even across large phylogenetic distances, and that discoveries of such distinct clades must be interpreted with caution. However, maintenance of ancient polymorphisms through selection is another plausible alternative that may cause the observed discordance and cannot be entirely excluded.Electronic supplementary materialThe online version of this article (10.1186/s12862-017-1070-4) contains supplementary material, which is available to authorized users.

Six Artemia populations, one bisexual and five parthenogenetic, from different parts of Iran were compared using morphometric and genetic characteristics. The discriminant analysis based on 19 morphometric variables showed that there are significant differences between the studied populations based on their morphological characteristics, where 85.9% of original grouped cases were correctly classified. The bisexual Artemia urmiana however exhibited a 100% separation from the parthenogenetic populations. However, a 1500 bp mitochondrial rDNA fragment showed similar RFLP patterns for all Iranian populations confirming earlier reports of a close genetic relationship between A. urmiana and parthenogenetic Artemia. IntroductionThe brine shrimp Artemia (Crustacea, Anostraca) is found abundantly in athalassohaline and thalassohaline environments at salinity levels ranging from 10 g/l (AGH et al., 2007) to 340 g/l (POST and YOUSSEF, 1977). The genus is a complex of bisexual and parthenogenetic species and probably superspecies defined by the criterion of reproductive isolation (BROWNE and BOWEN, 1991;PILLA and BEARDMORE, 1994). All bisexual species are diploid while asexual populations may be diploid, polyploid or a mixture of different ploidies (ABATZOPOU-LOS et al., 2003). Bisexual species are grouped as (1) the New World species: A. franciscana and A. persimilis and (2) the Old World species: A. salina, A. sinica, A. urmiana, A. tibetiana and Artemia sp. Kazakhstan (PILLA and BEARDMORE, 1994;ABATZOPOULOS et al., 2002b). The parthenogenetic forms are found only in the Old World and are grouped, rather controversially, under the binomen A. parthenogenetica (ABATZOPOULOS et al., 2002a, b). Populations of Artemia are found in more than 600 habitats distributed across the world in salt lakes and natural and man-made salterns ( VAN STAPPEN, 2002). Their distribution reflects the flight paths of some migratory birds and deliberate inoculations for commercial purposes by man (PERSOONE and SORGELOOS, 1980). AGH et al. (2002) and ABATZOPOULOS et al. (2006) reported Artemia populations from 17 biotopes in Iran, all parthenogenetic populations except for the bisexual Artemia urmiana. Due to the wide distribution of saline lakes and lagoons in Iran, the presence of Artemia in many geographic locations seems likely. Although the presence of Artemia in Urmia Lake was reported more than 100 years ago (GÜNTHER, 1899; see also ASEM, 2008), Iranian populations remained among the least studied populations so far.It is both due to the economic importance of Artemia and its critical role in larviculture of fish and shellfish that, in recent years, there has been a worldwide effort to discover new Artemia strains with specific responses to environmental conditions and to characterize them with regard to their potential use in aquaculture. Therefore, the characterization of Artemia populations and/or species has been a continuous endeavor since the second half of the previous century .Many scientists consider the genus Artemia as a c...