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.