The coexistence of different mating strategies, whereby a species can reproduce both by selfing and outcrossing, is an evolutionary enigma. Theory predicts two predominant stable mating states: outcrossing with strong inbreeding depression or selfing with weak inbreeding depression. As these two mating strategies are subject to opposing selective forces, mixed breeding systems are thought to be a rare transitory state yet can persist even after multiple speciation events. We hypothesise that if each mating strategy plays a distinctive role during some part of the species life history, opposing selective pressures could be balanced, permitting the stable co-existence of selfing and outcrossing sexual morphs. In this scenario, we would expect each morph to be specialised in their respective roles. Here we show, using 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. A. freiburgensis hermaphrodites are known to be produced under stressful conditions and are specialised for dispersal to new habitat patches. Here we show that they exhibit metabolic and intestinal changes enabling them to meet the cost of dispersal and reproduction. In contrast, A. freiburgensis females are produced in favourable conditions and facilitate rapid population growth. We found that females compensate for the lack of reproductive assurance by reallocating resources from intestinal development to mate-finding behaviour. The specialisation of each mating system for its role in the life cycle could balance opposing selective forces allowing the stable maintenance of both mating systems in A. freiburgensis.
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). Theory predicts only two stable mating states : outcrossing with strong inbreeding depression or selfing with weak inbreeding depression. 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. 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. In this scenario, we would expect each sexual morph to be specialised in their respective roles. 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. A. 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. In contrast, 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. 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.
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