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.