Sexual morph specialisation in a trioecious nematode balances opposing selective forces

Abstract

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.

De novo Genome Assembly of Auanema melissensis, a Trioecious Free-Living Nematode

“…A. freiburgensis hermaphrodites invest resources in the expansion of the intestine, the major metabolic organ, which may enable them to meet the high energy cost of dispersal and reproduction (Adams et al, 2022). In contrast, the obligate outcrossing female diverts resources from intestinal development to invest in mate-finding behavior (Adams et al, 2022). Here we show that the sexual morphs of A. melissensis exhibit similar developmental differences.…”

“…In A. freiburgensis, it has been proposed that females and hermaphrodites may play different roles in the life cycle, with each sexual morph exhibiting adaptations specific to their part, stabilizing trioecy in the population (Adams et al, 2022). A. freiburgensis hermaphrodites invest resources in the expansion of the intestine, the major metabolic organ, which may enable them to meet the high energy cost of dispersal and reproduction (Adams et al, 2022). In contrast, the obligate outcrossing female diverts resources from intestinal development to invest in mate-finding behavior (Adams et al, 2022).…”