The rapid dissolution of dioecy by experimental evolution

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Differential niche modification by males and females of a dioecious herb: extending the Jack Sprat effect

Males and females of dioecious plants often differ in morphological, physiological and life‐history traits, probably as a result of their different requirements for reproduction. We found that the growth and reproductive effort of individuals of the dioecious herb Mercurialis annua depended on whether males or females had been growing in the soil previously. This suggests that males and females of M. annua differentially modify the soil in which they are growing. Our study indicates that sexual dimorphism in dioecious plants can give rise to increased environmental heterogeneity as a consequence of sex‐specific niche modification.     

Sexually dimorphic eggs, nestling growth and sibling competition in American Kestrels Falco sparverius

1. American Kestrel ( Falco sparverius ) nestlings are sexually dimorphic, with daughters larger than sons. The larger daughters have an advantage during sibling competition for food in excess of their higher per capita food requirements, and we predicted that parents would reduce this competitive disparity by differentially enhancing the growth of sons, specifically by laying them in larger eggs.
2. In a captive breeding population, eggs producing sons were significantly larger than eggs producing daughters; laying order effects were controlled.
3. The influence of sibling egg size ratios on post-natal size relationships persisted through the nesting period, providing parents with a tool to manipulate size-related phenomena in their offspring.     

How does breeding system variation modulate sexual antagonism?

The study of sexually antagonistic (SA) traits remains largely limited to dioecious (separate sex), mobile animals. However, the occurrence of sexual conflict is restricted neither by breeding system (the mode of sexual reproduction, e.g. dioecy or hermaphroditism) nor by sessility. Here, we synthesize how variation in breeding system can affect the evolution and expression of intra- and inter-locus sexual conflicts in plants and animals. We predict that, in hermaphrodites, SA traits will (i) display lower levels of polymorphism; (ii) respond more quickly to selection; and (iii) involve unique forms of interlocus conflict over sex allocation, mating roles and selfing rates. Explicit modelling and empirical tests in a broader range of breeding systems are necessary to obtain a general understanding of the evolution of SA traits.     

Accessory male investment can undermine the evolutionary stability of simultaneous hermaphroditism

Sex allocation (SA) models are traditionally based on the implicit assumption that hermaphroditism must meet criteria that make it stable against transition to dioecy. This, however, puts serious constraints on the adaptive values that SA can attain. A transition to gonochorism may, however, be impossible in many systems and therefore realized SA in hermaphrodites may not be limited by conditions that guarantee stability against dioecy. We here relax these conditions and explore how sexual selection on male accessory investments (e.g. a penis) that offer a paternity benefit affects the evolutionary stable strategy SA in outcrossing, simultaneous hermaphrodites. Across much of the parameter space, our model predicts male allocations well above 50 per cent. These predictions can help to explain apparently ‘maladaptive’ hermaphrodite systems.     

Sex allocation in a simultaneous hermaphrodite, the blue-banded goby (Lythrypnus dalli): the effects of body size and behavioral gender and the consequences for reproduction

Although species with both male and female sexual functionsare often dichotomized into simultaneous and sequential hermaphrodites,many simultaneous hermaphrodites also exhibit sequential changesin sex allocation. In a field experiment using one such species,the gobiid fish, Lythrypnus dalli, female-biased individualsreallocated to male function in relation to their relative bodysize: consistent with the sizeadvantage hypothesis, large femaleswere more likely to reallocate and large fish had the highestspawningrates. Individuals, despite internal allocation to bothsexual functions, adopted only one behavioral gender. Behavioralmales had higher reproductive rates than behavioral females,and laboratory experiments showed that females preferred tomate with large males. Behavioral males grew more rapidly anddid notdiffer from behavioral females in survivorship. In addition,individuals who adopted male behavior but did not receive eggsin their nests maintained high levels of female tissue, whereasmales that received eggs did not. Laboratory experiments showedthat, unlike most hermaphroditic animals, L. dalli canchangeallocation either from ‘female’ to ‘male’or from ‘male’ to ‘female’. Thus, L.dalli shares haracteristics of both sequential and simultaneoushermaphrodites. Simultaneous hermaphroditism maybe maintained,in this species, to facilitate rapid sex change from femaleto male and to retain flexibility o that unsuccessful malescan revert to reproduction as females.