BioEssays 10/2020

Frequent independent origins of environmental sex determination (ESD) are assumed within amniotes. However, the phylogenetic distribution of sex-determining modes suggests that ESD is likely very ancient and may be homologous across ESD groups. Sex chromosomes are demonstrated to be old and stable in endothermic (mammals and birds) and many ectothermic (non-avian reptiles) lineages, but they are mostly non-homologous between individual amniote lineages. The phylogenetic pattern may be explained by ancestral ESD with multiple transitions to later evolutionary stable genotypic sex determination. It is pointed out here that amniote ESD shares several key aspects with sequential hermaphroditism of fishes such as a lack of sex differences in genomes, biased population sex ratios, and potentially also molecular mechanism related to general stress responses. Here, it is speculated that ESD evolves via a heterochronic shift of the sensitive period of sex change from the adult to the embryonic stage in a hermaphroditic amniote ancestor. Also see the video abstract here https://youtu.be/q2mjtlCefu4 .     

SEX-RATIO BIAS WITH ASYMMETRIC EXCHANGE OF POLLEN BETWEEN DEMES

In a patch of hermaphroditic plants, with a low level of pollen migration between patches, a prevailing wind creates a gradient, within the patch, in the strength of local competition among pollen for reproductive success. This leads to a sex ratio gradient, with a male (pollen) bias in downwind individuals, which can be quite strong even for large patches. The effect can be understood as follows: downwind individuals have relatively low reproductive value and respond by putting more resources into the gamete (pollen) with the best long-range (extra-patch) reproductive success.     

Sperm competition and sex allocation in simultaneous hermaphrodites

Summary Sex allocation theory is developed for hermaphrodites having frequent copulations and long-term sperm storage. Provided the sperm displacement mechanisms are similar to those known in insects, the ESS allocation to sperm versus eggs satisfies a rather simple rule. There are no data to test this rule, as yet.     

No correlation between inbreeding depression and delayed selfing in the freshwater snail Physa acuta

Inbreeding depression, one of the main factors driving mating system evolution, can itself evolve as a function of the mating system (the genetic purging hypothesis). Classical models of coevolution between mating system and inbreeding depression predict negative associations between inbreeding depression and selfing rate, but more recent approaches suggest that negative correlations should usually be too weak or transient to be detected within populations. Empirical results remain unclear and restricted to plants. Here, we evaluate, for the first time, the within-population genetic correlation between inbreeding depression and a trait that controls the amount of self-fertilization (the waiting time) in a self-fertile hermaphroditic animal, the freshwater snail Physa acuta. Using a large quantitative-genetic design (36 grand-families and 348 families), we observe abundant within-population family-level genetic variation for both inbreeding depression (estimated for survival, fecundity, and size) and the degree of behavioral selfing avoidance. However, we detected no correlation between waiting time and inbreeding depression across families. In agreement with recent models, this result shows that mutational variance rather than differential purging accounts for most of the genetic variance in inbreeding depression within a population.     

Association behaviour of the self-fertilizing Kryptolebias marmoratus (Poey): the influence of microhabitat use on the potential for a complex mating system

Preliminary observations were conducted to identify conspicuous body postures and movements of males and hermaphrodites in the mangrove killifish Kryptolebias marmoratus . These behaviours were used quantitatively to examine the social interactions for experimental pairings of K. marmoratus of different sexual states ( i.e. simultaneous hermaphrodite and male) in an aquarium with an open-water area and simulated crab burrows. This allowed observation of behaviours that could not be observed in the field. Kryptolebias marmoratus , regardless of sexual state and experimental treatment, spent 40% of the time in the burrow. Hermaphrodites exhibited a preference for associating with males rather than other hermaphrodites. The observed complexity of displayed behaviours and interactions between paired conspecifics indicate that K. marmoratus has a rich repertoire of social behaviour not predicted for a strictly selfing species. Also, land crab burrows play an important role in their social interactions.     

Disassociation between weak sexual isolation and genetic divergence in a hermaphroditic land snail and implications about chirality

Examination of the association between reproductive isolation and genetic divergence in a variety of organisms is essential for elucidating the mechanisms causing speciation. However, such studies are lacking for hermaphrodites. We measured premating (sexual) isolation in species pairs of the hermaphroditic land snail Albinaria and we compared it with their genetic divergence. We did not find substantial sexual isolation barriers between the species studied. The absence of strong sexual isolation between species implies its minor effect in the evolution of this genus, because distributional, population and life-history characteristics of Albinaria make mate-choice possibly redundant. Furthermore, we found disassociation between genetic divergence and sexual isolation, suggesting that they do not form necessarily a cause-effect duet. However, Albinaria voithii, the only dextral Albinaria species, shows strong sexual isolation against the other sinistral species. We discuss whether change in coiling either has triggered instantaneous speciation, or is an example of character displacement.     

Allocation of resources to male and female functions in hermaphrodites

The question of"how a self-fertile hermaphrodite will distribute the resources that it allocates to reproduction is studied by means of the ESS approach. Different models of the relations between allocation to male function, the male and female fertilities, and the selfing rate, yield different conclusions about how much resource should be allocated to male function. Values below a half are obtained with one model, while another can give values greater than a half. Even with no selfing, values other than a half are usually obtained; with both models studied, the values decrease with increasing selling. If the selfing rate is assumed to be independent of the fraction of resources allocated to male function, it can be shown that the ESS allocation to male function always decreases as selling increases. The types of relations that might be expected in species with different types of breeding biology, and some data on allocation to male function, are reviewed.
The implications for the fitness of male- and female-sterility mutations are discussed. It is argued that the concavity or convexity of the curve relating female fertility to male fertility is not a good guide to when hermaphroditism should exist when there is some selfing. Even with a concave relation, male-sterility mutants can have a higher fitness than hermaphrodites, if there is some selling and inbreeding depression. Also, when the selfing rate depends on allocation to male I unction, an hermaphrodite ESS does not always exist when the function is concave (as it does when there is no selfing), and such an ESS may exist when the relation is convex. The fitness of male- or female-sterility mutants may also depend on the existence of 'fixed costs'. It is shown that these do not ailed the ESS allocation of resources.     

Multiple mating increases cocoon hatching success in the earthworm Eisenia andrei (Oligochaeta: Lumbricidae)

In simultaneous hermaphrodites with reciprocal mating, multiple mating may be a male strategy that conflicts with female interests, and therefore an intra‐individual sexual conflict regarding the number of matings may be expected. The evolutionary outcome of this sexual conflict will depend on the costs and benefits that extra mating entails for each sexual function. In the present study, we investigated the costs and benefits of multiple mating on cocoon number, cocoon mass, and cocoon hatching success in the redworm Eisenia andrei, a simultaneous hermaphrodite with reciprocal insemination, by manipulating the number of matings with different partners. We did not detect any reduction in the female reproductive output (number and mass of cocoons) with increasing number of mating partners. However, we found that multiple mating showed benefits for female reproduction that increased the hatching success of the cocoons. This effect may be a result of increased quantity and/or diversity of sperm in the spermathecae of multiple mated earthworms. Further studies are required to clarify the mechanism underlying the increased cocoon hatching success when redworms engage in multiple matings. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Self‐fertilization and inbreeding limit the scope for sexually antagonistic polymorphism

Sexual antagonism occurs when there is a positive intersexual genetic correlation in trait expression but opposite fitness effects of the trait(s) in males and females. As such, it constrains the evolution of sexual dimorphism and may therefore have implications for adaptive evolution. There is currently considerable evidence for the existence of sexually antagonistic genetic variation in laboratory and natural populations, but how sexual antagonism interacts with other evolutionary phenomena is still poorly understood in many cases. Here, we explore how self‐fertilization and inbreeding affect the maintenance of polymorphism for sexually antagonistic loci. We expected a priori that selfing should reduce the region of polymorphism, as inbreeding reduces the frequency of heterozygotes and speeds fixation. This expectation was supported, but although previous results suggest that the more an allele that is deleterious to one sex is dominant in that sex, the smaller the region of parameter space that will admit polymorphism, we found that this effect is weakened by self‐fertilization. However, the effect of inbreeding is not strong enough to completely cancel out the effect of dominance: For a given frequency of inbreeding, it will still be the case that the more dominant the alleles are in their deleterious context, the smaller the region of parameter space in which they can exist at polymorphism.