3.2. Transition to asexuality
While we discussed speciation of daphniids in the scope of reproductively isolated groups, we would be remiss to neglect the role of speciation in obligately asexual daphniids (see Glossary). Asexual species can be defined as distinct groups capable of adapting to different ecological niches due to divergent selection or geographic isolation (Barraclough, Birky Jr., & Burt, 2003; Birky Jr., Wolf, Maughan, Herbertson, & Henry, 2005). The transition to obligate asexuality has been studied extensively within the D. pulexspecies complex, particularly in hybrids of D. pulex and D. pulicaria. Obligately parthenogentic (OP) populations of Daphniaoften have a hybrid background (D. pulex x D. pulicaria ) and are found in northeastern and southern North America in intermediate habitats such as permanent ponds (Pantel, Juenger, & Leibold, 2011) and can be also found in the central region of North America along with cyclically parthenogenic (CP) populations (Hebert et al., 1993; Hebert & Finston, 2001). The transition to OP has been traced back to roughly 1-175 kya (Hebert et al., 1989; Crease, Stanton, & Hebert, 1989; Paland, Colbourne, & Lynch, 2005; Tucker, Ackerman, Eads, Xu, & Lynch, 2013), suggesting a relatively recent evolution of obligate asexuality.
The switch from CP to OP in D. pulex can be considered as a form of hybrid sterility, as a result of Dobzhansky-Muller incompatibilities. OP is caused by a cluster of meiosis suppressor genes that are located in at least four different genomic regions that interact epistatically (Lynch, Seyfert, Eads, & Williams, 2008; Tucker et al., 2013; Xu, Innes, Lynch, & Cristescu, 2013). Many of these alleles are found to be of D. pulicaria origin (Xu, et al., 2013). As OP populations have not been reported in D. pulicaria (but see Culver & Acosta, 2018; Millette et al., 2020), it is hypothesized that OP originated due to an ancient hybridization event between D. pulexand D. pulicaria , followed by introgression via multiple rounds of backcrossing (Tucker et al., 2013; Xu et al., 2013; Xu et al., 2015). Despite lower male production in OP populations (Innes, Fox, & Winsor, 2000), some OP females can produce males that can undergo spermatogenesis (Innes & Hebert, 1988; Hebert et al., 1989), as meiosis is not suppressed in males. These males can mate with sexual females in the population, thereby spreading obligate asexuality to sexual populations, a process termed ‘contagious asexuality’ (Innes & Hebert, 1988; Lynch et al., 2008). Another case of hybrid sterility due to Dobzhansky-Muller incompatibilities is the non-male producing clones observed within populations of D. pulex (Innes & Dunbrack, 1993). These non-male producers have a genetic basis which can be traced back to introgression with D. pulicaria (Ye et al., 2019).
While theory suggests that asexual lineages are considered evolutionary dead ends because they accumulate a high mutation load (e.g. Muller’s ratchet), high rates of loss of heterozygosity events (a form of ameiotic recombination) have been documented in obligate asexual lineages (Omilian, Cristescu, Dudycha, & Lynch, 2006; Xu, Omilian, & Cristescu, 2011). Moreover, OP populations that produce males can cross with CP populations and can account for the persistence of these lineages in nature (Paland et al., 2005). Other cases of asexuality have been documented in European populations of D. pulex (Lehto & Haag, 2010), D. magna (Svendsen et al., 2015) and D. cephalata (Hebert, 1981); however, little is known about the underlying mechanism involved in the transition to obligate asexuality in these species, and how these OP populations persist in nature.