2.2.2. Behavioral and non-ecological isolation
Behavioural isolation can be a strong reproductive isolating barrier for species with complex mating signals and rituals. Also known as ethological isolation, this reproductive barrier can occur when closely related species do not mate due to differences in their complex courtship rituals. Behavioural isolation has been found to be a strong barrier between closely related species of rotifers (Schroder & Walsh, 2010). However, the role of behavioural isolation in the speciation ofDaphnia received very little attention.
Fundamental aspects of the mating behaviour of daphniids remain largely unexplored. For example, it is unclear how males locate and recognize conspecific females. In copepods and rotifers, males use chemoreception, and it is suggested that males can discriminate the pheromones of conspecific females (Katona, 1973). In Daphnia , males are found to pursue sexually receptive females more often than unreceptive females (Winsor & Innes, 2002), suggesting some form of recognition of the reproductive stage of females. Additional evidence comes from swarming behaviour during sexual reproduction in Daphnia (Young, 1978). Furthermore, swimming patterns between males and females are distinct, specifically, males swim faster and orthogonally, a pattern known as ‘scanning’ behaviour (Gerritsen, 1980; Brewer, 1998) that can maximise encounter rate. Yet, Crease and Hebert (1983) were unable to replicate swarming behaviour in a laboratory setting and determine whether males were capable of detecting females through chemoreception. An assessment of the D. pulex reference genome revealed 58 gustatory chemoreceptor genes (Penalva-Arana, Lynch, & Robertson, 2009), which may be good subjects for research on mate recognition. Another proposed strategy for males to locate females is via mechanoreception of fluid disturbances generated by the specific swimming and filtering behaviour (Brewer, 1998). It is possible that glycoproteins located in the ovaries of females could provide a distinguishing signal for mate recognition once the male attaches to the female (Carmona & Snell, 1995). However, it has not been shown that either of these strategies are commonly used for mate recognition in daphniids.
Daphnia mating behaviours were first documented by Jurine (1820) and Weismann (1880) who concluded that Daphnia do not exhibit elaborate courtship rituals after males locate females. More recently, mating behaviours have been described in a few species of daphniids (Brewer, 1998; Winsor & Innes, 2002; La et al., 2014). Once the male attaches to the female, copulation occurs roughly 15 seconds before the male detaches (Brewer, 1998). While males appear to mate indiscriminately when they encounter females, females are found to exhibit behaviours to deter mating (e.g. escape response), suggesting female choice (Jurine, 1820). While Winsor and Innes (2002) have examined mate choice in conspecific D. pulex , mate choice experiments between closely related species have largely not been explored, and little is known about whether there are differences in mating behaviours across species of daphniids.
In the absence of ecological isolating barriers, hybridization between closely related species of daphniids is common (Schwenk & Spaak, 1995; Keller et al., 2008), suggesting that behavioural isolation may not be a strong reproductive barrier in this genus. However, not all sister species are found to hybridize indiscriminately when in contact, and it is not fully understood if daphniids can exhibit other subtle forms of mating recognition. Recently, no-choice mating experiments betweenD. pulex and D. pulicaria have found lower mating-fertilization success in heterospecific than in conspecific crosses (Chin, Cáceres, & Cristescu, 2019). Mate choice experiments between closely related species are needed to determine the degree to which daphniids show preference for conspecifics in the presence of heterospecific mates.