However, the large and diverse phylum Nematoda provides an opportunity to study the evolution and potential stability of mixed mating strategies and its relationship to specific life histories. To date, the study of sexual polymorphisms has primarily focused on flowering plants. We would expect a stable mixed mating system to exist only where these opposing selective forces are balanced. to reduce inbreeding or in response to selection for sexual morph specialization, reviewed in ( Charlesworth, 1999). Otherwise, dioecy (females and males) should dominate if selection for reproductive assurance is relaxed, e.g.

In a transitory system, self-fertilising hermaphrodites should outcompete females and males if there is strong selection for reproductive assurance (guaranteeing reproduction even if a male is not available) (Pannell, 1997a Wolf & Takebayashi, 2004). Mixed mating strategies are expected to be rare, as they are subject to opposing selective forces. Yet mixed breeding have been found to persist even after multiple speciation events ( Weeks, 2006), suggesting that in some cases they are not transitory. Species with such mixed mating strategies are usually considered a temporary transitional state, as theory predicts only two stable mating states: outcrossing with strong inbreeding depression or selfing with weak inbreeding depression ( Charlesworth, 1984 Charlesworth et al., 1990 Lande and Schemske, 1985). The coexistence of different mating strategies within a species, where conspecifics can reproduce either by outcrossing (male and female) or self-fertilising (hermaphrodites), is an evolutionary enigma that has long intrigued biologists ( Darwin,1877). Natural selection has driven the evolution of diverse modes of reproduction, ranging from species that replicate exclusively from a single parent to those that have separate sexes. The specialisation of each mating system for their role in the life cycle could balance opposing selective forces allowing the stable maintenance of both outcrossing and selfing mating systems in A. freiburgensis females are produced in favourable conditions, facilitate rapid population growth and compensate for the lack of reproductive assurance by reallocating resources from intestinal development to robust mate-finding behaviour. freiburgensis hermaphrodites are produced under stressful conditions, are specialised for dispersal to new habitat patches and exhibit metabolic and intestinal changes that enable them to meet the energetic cost of dispersal and reproduction. Here we show, using a combination of behavioural, physiological and gene expression studies, that the selfing (hermaphrodite) and outcrossing (female) sexual morphs of the trioecious nematode Auanema freiburgensis have distinct adaptations optimised for their different roles during the life cycle. In this scenario, we would expect each sexual morph to be specialised in their respective roles. We hypothesise that if each mating strategy plays a distinctive role during the species life history, opposing selective pressures could be balanced, permitting the stable co-existence of selfing and outcrossing sexual morphs.

As these two mating strategies are subject to opposing selective forces, mixed breeding systems are thought to be a rare transitory state, yet they have been found to persist even after multiple speciation events. Theory predicts only two stable mating states : outcrossing with strong inbreeding depression or selfing with weak inbreeding depression. The coexistence of different mating strategies, whereby a species can reproduce both by selfing and outcrossing, is an evolutionary enigma that has long intrigued biologists ( Darwin, 1877).

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