Genetic characterization of breeding systems, ploidy levels and species boundaries in Cypricercus (Ostracoda)

Abstract

Although freshwater ostracod crustaceans show an exceptionally high incidence of transitions from sexual reproduction to parthenogenesis, no prior studies have examined genetic relationships among congeneric species showing breeding system variation. The present study addresses this gap by examining patterns of allozyme diversity among three unisexual and three sexual species in the genus Cypricercus from 46 North American sites. Patterns of genetic relatedness among these taxa suggest that parthenogenesis has evolved on at least three occasions within the genus, and that one all-female taxon is closely related to a sexual species. One unisexual, C. reticulatus, showed very high clonal diversity with many polyploids as well as an allelic array suggesting that much of its diversity originated through interbreeding with sexual relatives. The other unisexuals showed both lower clonal and allelic diversity, but the prevalence of triploid clones suggested that variation might originate from hybridization between diploid asexuals and males of relict sexual populations. The prevalence of multiple X-Y sex determination systems in the ostracods may explain the marked variation in chromosome numbers evident among polyploids. This study suggests that the success of unisexuals may be linked to their ability to recruit clonal diversity through interbreeding with close sexual relatives.

Heterocypris(Crustacea, Ostracoda) from the Isole Pelagie (Sicily, Italy): Population genetics

“…In fact, asexual-sexual hybridisation is known as an important mechanism in increasing clonal diversity: Turgeon and Hebert (1994) described two triploid clones of Cyprinotus (^Hetero-cypris) incongruens and suggested that they are hybrid between a diploid parthenogenetic female of C. incongruens and a male of the syngamic species C. glaucus. In Cypricercus {= Bradleystrandesid) fuscatus, the origin of different polyploid clones was related to mating between diploid parthenogenetic females and males of C. tincta (Turgeon & Hebert, 1995). Persistence of asexuality and clonal differentiation was explained by intraspecific hybridisation between sympatric males and parthenogenetic females of Eucypris virens (Rossi et al, 1998;Schön et al, 2000;Rossi et al, in preparation).…”

“…On the other hand, it is known that in apomictic parthenogens, genetic diversity increases through mating with close sexual relatives or rare males (Pernin et al, 1992;Turgeon & Hebert, 1994, 1995Schön etal, 2000).…”

“…In North America, the genus Cyprinotus (=Heterocypris) contains 15 species, nine of which are sexual (Turgeon & Hebert, 1994). For most species, and in particular for H. incongruens (Ramdhor, 1808), both parthenogenetic and amphimictic populations are known, and interspecific hybridisation is described as a mechanism for increasing clonal and genetic variability (Turgeon & Hebert, 1995). The taxonomy of the group can be difficult to interpret due to its wide distribution, passive dispersal and morphological plasticity which often is related to chemical and physical factors (Purper & Wuerdig-Maciel, 1974;Sywula & Lorenc, 1981;Benzie, 1984).…”

Inbreeding and outbreeding depression in geographical parthenogensHeterocypris incongruensandEucypris virens(Crustacea: Ostracoda)

“…The most established viewpoint is that this pattern has evolved due to the higher colonisation ability of asexuals ). On the other hand, the frequent transition from sexual to asexual reproduction and the interbreeding with sexual congeners explain the persistence of asexuality, the generation of polyploid lineages and the high clonal diversity reported in nonmarine ostracods (Havel & Hebert 1989;Chaplin et al 1994;Little & Hebert 1994;Turgeon & Hebert 1994;Turgeon & Hebert 1995;Chaplin & Hebert 1997;Little & Hebert 1997;Schön et al 2000;Cywinska & Hebert 2002;Little 2005;Rossi et al 2006;Rossi et al 2008;Adolfsson et al 2009). In general, populations of sexual species and sexual populations of geographic parthenogens showed agreement of genotype frequencies with HardyWeinberg expectations at single loci, random associations at pairs of loci and little evidence of inbreeding was reported (Havel et al 1990;Little & Hebert 1994;Turgeon & Hebert 1995;Finston 2002).…”

Spatial segregation of Darwinula stevensoni (Crustacea: Ostracoda) genotypes in lentic and lotic habitats of Northern Italy

“…Many extant geographic parthenogens are much more widespread than their sexual relatives, present very high clonal diversity and specialised ecotypes capable of existence and coexistence under a wide variety of environmental conditions Rossi et al 1998;Cywinska & Hebert 2002). In Cypricercus, Heterocypris (Cyprinotus) and Eucypris, the recruitment of new clones is probably obtained through interbreeding among congeneric species or close sexual relatives (Turgeon & Hebert 1994, 1995Rossi et al 1998;Schön et al 2000).…”