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.