Reproduction and genetic variation in clam shrimps (Crustacea, Branchiopoda, Conchostraca)

Samples of the three clam shrimp species. Eoleptestheria ticinensis and Leptestheria dahalacensis (Leptestheriidae) and Limnadia lenticularis were collected in north Italian rice paddies in Emilia and Lombardy. Sex-ratio analysis revealed that in the two species of Leptestheriidae, males are more frequent than females, whereas L. lenticularis has an all-female constitution. Allozyme analysis showed a low degree of genetic variation, the values of polymorphism parameters being very similar to those found for other Branchiopoda. In the Leptestheriidae it was generally found that genotypic frequencies conform to Hardy-Weinberg expectations, demonstrating a high outcrossing rate. No clear-cut relationship between the excess of males and cytological and genetic observations could be established, since neither sex chromosomes nor sex-linked genes were found. Nevertheless, male predominance could overcome the morphodynamic constraints that clam shrimps face during fertilization. The all-female constitution of the two populations of L. lenticularis and the hatching of unisexual progeny from virgin females suggest that they reproduce by obligate parthenogenesis; the almost complete lack of heterozygous genotypes at all the analyzed loci allows us to discard the previous hypothesis of automixis. Further analyses should be undertaken to fully clarify the sex-determination mechanisms operating in these clam shrimps, as was previously done in Eulimnadia texana.