2.2.4. Postzygotic isolation
Postzygotic isolation occurs when hybridization between closely related
species produces unfit hybrids due to intrinsic (developmental) or
extrinsic (environmental) factors. In the Daphnia genus,
interspecific hybridization is common, with closely related species in
all species complexes producing hybrids with one potential exception,
the Daphnia longiremis species complex. Interspecific
hybridization was long suspected to occur due to the presence of
intermediate morphological forms (Einsle, 1966; Lieder, 1983; Flossner
& Kraus, 1986; Taylor & Hebert, 1992; Giessler, Mader, & Schwenk,
1999) and this observation was validated by molecular analyses (Wolf &
Mort, 1986; Wolf, 1987; Taylor & Hebert, 1993; Spaak, 1996; Spaak,
1997; Giessler, 1997; Giessler & Englebrecht, 2009). Detailed studies
on hybrids revealed intermediate life history attributes (Weider &
Wolf, 1991; Spaak & Hoekstra, 1995; Dudycha & Tessier, 1999),
differences in resource utilization (Boersma & Vijverberg, 1994a;
Repka, 1996; Von Elert, 2004) and other ecological requirements (Taylor
& Hebert, 1992; Duffy et al., 2004). Here, we summarize the potential
of postzygotic isolating barriers in Daphnia based on intrinsic
and extrinsic postzygotic isolation.
Intrinsic postzygotic isolation occurs as a result of genetic
differences between closely related species that diminish the production
and/or performance of hybrids and can occur either during the
developmental process (hybrid inviability), or during sexual maturity
(hybrid sterility). Three closely related species, D. galeata ,D. longispina (hyalina morph), and D. cucullata are
found to co-occur in the same area or habitats, and hybridization
between these three species is often bidirectional (Giessler et al.,
1999). Introgression of alleles from one species to another via
hybridization and backcrossing has also been documented in many studies
(Taylor & Hebert, 1992; Taylor & Hebert, 1993; Schierwater, Ender,
Schwenk, Spaak, & Streit, 1994; Spaak, 1996; Spaak, 1997; Spaak, Denk,
Boersma, & Weider, 2004; Yin, Wolinska, & Giessler, 2010). Field
experiments found low hatching success and survivorship of hybrids
compared to parental species (Carvalho & Wolf, 1989; Wolf & Carvalho,
1989; Keller & Spaak, 2004; Griebel, Giessler, Yin, & Wolinska, 2016).
Laboratory hatching experiments between D. galeata and D.
cucullata, and D. galeata and D. longispina(hyalina morph) corroborate these findings (Schwenk, Bijl, &
Menken, 2001; Keller & Spaak, 2004; Brede, Straile, Streit, & Schwenk,
2007; Keller, Wolinska, Tellenbach, & Spaak, 2007). Asymmetric
reproductive isolating barriers are also observed. For example, crosses
between D. cucullata females and D. galeata males are more
successful than the reciprocal crosses, consistent with observations of
these particular hybrids commonly found in nature (Schwenk et al.,
2001). While unidirectional crosses between D. pulex females withD. pulicaria males show higher hatching and survivorship than
parentals (Heier & Dudycha, 2009), postzygotic isolating barriers were
found to be symmetric between D. pulex and D. pulicaria(Chin et al., 2019).
Extrinsic postzygotic isolation occurs due to ecological or behavioural
differences between closely related species, which reduces the
performance of hybrids that often have intermediate characteristics. In
nature, hybrids are locally produced and occur in hybrid zones with
environmentally favourable conditions (Taylor & Hebert, 1992; Muller &
Seitz, 1995; Spaak, 1994; Spaak, 1997). Persistence of hybrids in the
environment may be consistent with the hybrid inferiority model, where
hybrids occur in ‘tension zones’ and experience the balancing forces of
dispersal and selection against hybrids (Barton & Hewitt, 1985).
However, since Daphnia hybrids can propagate by clonal
reproduction, hybrid persistence and dominance can be observed in
‘tension zones’ (Weider, 1993; Spaak & Hoekstra, 1997; Schwenk, 1997),
leading to the temporal hybrid superiority model (Spaak & Hoekstra,
1995). Examples of temporal hybrid superiority are relatively common inDaphnia , where hybrids are found to exhibit higher fitness than
the parental species in certain environmental conditions and periods of
the year, such as food quality (Seidendorf, Boersma, & Schwenk, 2007;
Brzezinski & Von Elert, 2007; Weider, Jeyasingh, & Looper, 2008) and
concentration (Boersma & Vijverberg, 1994b; Repka, Vesela, Weber, &
Schwenk, 1999), predation (Declerck & De Meester, 2003), temperature
(Weider & Wolf, 1991), parasitism (Wolinska, Bittner, Ebert, & Spaak,
2006) and a variety of other environmental factors (Griebel et al.,
2015). However, hybrid fitness often varies in different environments
(Loffler, Wolinska, Keeler, Rothhaupt, & Spaak, 2004). Ecological
inviability of hybrids has been hypothesized to occur between hybrids ofD. cucullata and D. longispina (hyalina morph),
where hybrids often occur in intermediate habitats compared to parentals
(Seda, Petrusek, Machacek, & Smilauer, 2007; Petrusek et al., 2008),
exhibit intermediate traits for foraging (Machacek & Seda, 2016), and
lower fitness in response to predation pressure (Spaak, Vanoverbeke, &
Boersma, 2000). Hybrids of D. cucullata and D. galeatadisplay intermediate traits linked to predator avoidance (Spaak, 1995).
Moreover, overwintering strategies also differ between D. galeataand D. longispina (hyalina morph), where D. galeataoverwinters, D. longispina (hyalina morph) produces
resting eggs, and hybrids exhibit intermediate traits (Zeis, Horn,
Gigengack, Koch, & Paul, 2010). D. galeata x D.
longispina (hyalina morph) hybrids are infected by parasites
more often than parental species, thereby reducing their fitness
(Wolinska, Keller, Bittner, Lass, & Spaak, 2004). Additionally, low
fitness was observed in these hybrids under low food quality
(Brzezinski, 2015).
While life history experiments to determine the role of ecology in
hybrids in Daphnia are common, the potential involvement of
behavioural sterility in hybrids remains largely unexplored. Detailed
observations of mating behaviour of hybrids compared with parental
species would greatly benefit this line of research. Experimental case
studies are particularly needed to determine the extent of postzygotic
isolating barriers across different sister species of Daphnia .
Such studies are particularly lacking in species from the southern USA
and Mexico, where endemic daphniid species inferred from allozyme data
(Hebert, Schwartz, Ward, & Finston, 1993; Hebert & Finston, 1996)
produce hybrids detected in intermediate habitats compared to those
occupied by the parental species.