3.1 Hybridization and polyploidization
Hybridization is commonly found in nature (Arnold, 1997; Mallet, 2005),
and yet the role of hybridization and gene flow on speciation remains
contentious. On one hand, gene flow can be viewed as a detriment to the
speciation process due to its homogenization effects on locally adapted
alleles and the dismantling effect in emerging reproductive barriers
(Mayr, 1963). And yet, gene flow can result in the production of new
species (e.g. instantaneous speciation) by strengthening prezygotic
reproductive barriers due to reinforcement (Slatkin, 1987; Rieseberg,
1997; reviewed in Abbott et al., 2013). Theoretical models have shown
that under some scenarios, gene flow can aid the process of speciation
(Kondrashov & Kondrashov, 1999; Pinho & Hey, 2010). These theoretical
predictions are supported by increasing evidence of
speciation-with-gene-flow occurring in nature (Niemiller, Fitzpatrick,
& Miller, 2008; Via, 2009; Rougeux, Bernatchez, & Gagnaire, 2017).
Hybrid persistence occurs in Daphnia due to continuous local
production and clonal propagation under favourable conditions. In theDaphnia genus, a well documented instance of hybridization giving
rise to a new species is the Daphnia middendorffiana case.Daphnia middendorffiana is primarily distributed in the Arctic
regions and is believed to be the result of hybridization betweenD. pulicaria and D. tenebrosa or D. melanicasometime during the Pleistocene glaciations (Dufresne & Hebert, 1997).D. middendorffiana has a high tolerance to lower temperatures
(Yurista, 1999), and can show stronger pigmentation, which was
hypothesized to be an adaptive trait against UV radiation (Luecke &
O’Brien, 1983). While less explored, speciation has also been suggested
to arise in North American D. mendotae from reticulate events
between European populations of D. galeata and D.
longispina (Taylor et al. 1996).
Interspecific hybridization in Daphnia can also result in
polyploidy. Parthenogenesis has been found to be a prerequisite to
polyploidy in plants (Stebbins, 1950), and this also appears to be the
case for parthenogenic organisms such as Daphnia . Daphniid
polyploids are generally triploids and tetraploids as a result of genome
duplication (autopolyploidization), hybridization
(allopolyploidization), or a combination of both. In crosses betweenD. pulex and D. pulicaria , the production of polyploids
appears to depend on the direction of the cross. When D.
pulicaria is the mother, polyploids are produced; yet diploids are
formed when D. pulex is the maternal parent (Dufresne & Hebert,
1994; Dufresne & Hebert, 1997). These polyploids can often be found in
arctic and subarctic regions, with increasing latitudes exhibiting an
increase in the occurrence of polyploids (geographical parthenogenesis;
Beaton & Hebert, 1988; Ward, Bickerton, Finston, & Hebert, 1994).
In the temperate regions of South America, asexual tetraploids are often
identified based on molecular analyses (Adamowicz, Gregory, Marinone, &
Hebert, 2002). Phylogenetic analyses showed that these temperate
polyploids originated from a common ancestor that is likely related to
North American D. pulicaria. It has been suggested that the
common ancestor of these lineages was a hybrid form of D. pulex xD. pulicaria , which may have been introduced to South America
(Mergeay et al., 2008). Asexual tetraploids are also found in
high altitude lakes in the tropical Andes region (Aguilera, Mergeay,
Wollebrants, Declerck, & De Meester, 2007). In contrast to the
temperate polyploids in Argentina, these tropical polyploids are likely
of local origin, as evident by a high degree of genetic variation,
comparable to the Arctic polyploids.
It has been hypothesized that polyploidy is selected for extreme
environments due to its high adaptive potential (Beaton & Hebert, 1988;
Van de Peer, Mizrachi, & Marchal, 2017). In the case of Arctic
polyploids, clonal diversity is higher than in the diploid counterparts,
which can promote phenotypic diversity due to beneficial mutations,
leading to an increase in fitness (Weider, Beaton, & Hebert, 1987). For
example, some Arctic Daphnia polyploids exhibit melanism, which
protects against UV radiation and are capable of inhabiting clear-water
ponds unlike their non-melanic counterparts, which inhabit only ponds
with high humic content (Hebert & McWalter, 1983; Weider & Hebert,
1987). Additionally, polyploids were found to have divergent
physiological strategies in osmoregulation depending on their habitats
(Weider & Hebert, 1987). Polyploids were also found to reach maturity
at a later stage compared to diploids, producing larger body sizes at
maturity and smaller clutch sizes with larger-sized offspring (Weider,
1987). Life-history comparisons between polyploid and their diploid
counterparts found polyploids to be better adapted and have earlier
maturation at lower temperatures and are therefore better suited for the
short reproductive season in the Arctic (Dufresne & Hebert, 1998).
Another advantage of polyploids is that the additional genetic material
can reduce the effects of accumulating recessive deleterious mutations,
as daphniid polyploids are typically obligately parthenogenic (OP).
Unlike their sexual diploid counterparts, the obligately asexual
polyploids cannot purge the mutational load and are theorized to
represent evolutionary dead ends.