Successful mating and hybridisation in two closely related flatworm species despite significant differences in reproductive morphology and behaviour

Abstract

Reproductive traits are some of the fastest diverging characters and can serve as reproductive barriers. The free-living flatworm Macrostomum lignano , and its congener M. janickei are closely related, but differ substantially in their male intromittent organ (stylet) morphology. Here, we examine whether these morphological differences are accompanied by differences in behavioural traits, and whether these could represent barriers to successful mating and hybridization between the two species. Our data shows that the two species differ in many aspects of their mating behaviour. Despite these differences, the species mate readily with each other in heterospecific pairings. Although both species have similar fecundity in conspecific pairings, the heterospecific pairings revealed clear postmating barriers, as few heterospecific pairings produced F1 hybrids. These hybrids had a stylet morphology that was intermediate between that of the parental species, and they were fertile. Finally, using a mate choice experiment, we show that the nearly two-fold higher mating rate of M. lignano caused it to mate more with conspecifics, leading to assortative mating, while M. janickei ended up mating more with heterospecifics. Thus, while the two species can hybridize, the mating rate differences could possibly lead to higher fitness costs for M. janickei compared to M. lignano .

Large-scale phylogenomics of the genus Macrostomum (Platyhelminthes) reveals cryptic diversity and novel sexual traits

“…These internal nodes were separated by short branches suggestive of rapid speciation events, such as during adaptive radiation (Irisarri et al 2018), where substantial incomplete lineage sorting is expected. This pattern is also consistent with ancient hybridization, which is a distinct possibility, since there is evidence for hybridization between the sibling species M. lignano from the Eastern Mediterranian and M. janickei from the Western Mediterranian under laboratory conditions (Singh et al 2020).…”

“…The second clade (called "Clade II" in Schärer et al 2011) contains species that are thought to mate primarily through reciprocal copulation, where both individuals reciprocally insert their (often blunt) stylet (Fig. 1A) into the female antrum of the mating partner, and are able to donate and receive ejaculate at the same time (see also Schärer et al 2004Schärer et al , 2011Singh et al 2020). Within Clade II one species, M. hystrix, was found to have convergently evolved the hypodermic mating behaviour, and this was associated with convergent changes in stylet, sperm, and antrum morphology, leading to a morphology that is very similar to that of M. pusillum (Fig.…”

Frequent origins of traumatic insemination involve convergent shifts in sperm and genital morphology

“…These internal nodes were separated by short branches suggestive of rapid speciation events, such as during adaptive radiation [73], where substantial incomplete lineage sorting is expected. This pattern is also consistent with ancient hybridisation, which is a distinct possibility, since there is evidence for hybridisation within the genus under laboratory conditions [74].…”

“…During reciprocal copulation two worms insert their—often relatively blunt—stylet (Figure 1) via their partner’s female genital opening into the female sperm storage organ, the female antrum (further called antrum), so that both can donate and receive sperm in the same mating [50]. Many reciprocally copulating species perform a postcopulatory suck behaviour, where worms place their mouth over their own female genital opening and suck, presumably in an attempt to remove components of the received ejaculate from their antrum [45, 50–53]. This ejaculate removal could target manipulative seminal fluids, since the ejaculate of the model species M. lignano, contains substances affecting the mating partner’s propensity to perform the suck behaviour [54, 55].…”

Faster rates of molecular sequence evolution in reproduction-related genes and in species with hypodermic sperm morphologies

“…Macrostomum are simultaneous hermaphrodites, producing sperm and eggs at the same time, and several species from this genus, particularly the main model species M. lignano (Ladurner et al 2005;Wudarski et al 2020), are used to study mating behaviour, sexual selection, and sexual conflict (e.g. Schärer et al 2005;Ramm et al 2015;Marie-Orleach et al 2017;Giannakara et al 2016;Giannakara and Ramm 2017;Patlar et al 2020;Singh et al 2020aSingh et al , 2020b. In hermaphrodites, sexual conflict is expected to occur not just over the mating and remating rates, but also over the sex roles that the partners assume during copulation, with the male (or "sperm donor") role generally thought to be preferred over the female (or "sperm recipient") role (Charnov 1979;Michiels 1998;Schärer et al 2015).…”