The evolution of sex determining systems

Sex determination systems are highly variable across taxa. Heterogamety has already been mentioned. Actually, this is a general term that conceals much underlying genetic diversity. Males are the heterogametic sex in mammals, females in birds, but reptiles, amphibians, and fish display both variants, often within the same family. Different genetic systems are even known to underlie male heterogamety: in mammals dominant male determiners exist on the Y chromosome. In common sorrel plants, the Y chromosome is inert, and gender is determined by the ratio of X chromosomes to autosomes. In the Hymenoptera and some other arthropods, haplodiploidy occurs, whereby males are haploid, developing either from unfertilized eggs or from

Condition

Fig. 5.6 The Trivers-Willard hypothesis of conditional sex expression. In organisms with polygynous mating systems (mammals, some fish), males benefit more from good condition than females (top). Therefore selection favours male production under good conditions and female production under poor conditions (bottom). In other organisms (most plants, many insects), females benefit more from good conditions and sex allocation is reversed.

Condition

Fig. 5.6 The Trivers-Willard hypothesis of conditional sex expression. In organisms with polygynous mating systems (mammals, some fish), males benefit more from good condition than females (top). Therefore selection favours male production under good conditions and female production under poor conditions (bottom). In other organisms (most plants, many insects), females benefit more from good conditions and sex allocation is reversed.

loss of the paternal genome after fertilization. Females are 'normal' diploids. In addition to such 'genetic' systems are the so-called 'environmental' systems whereby key variables experienced during development determine gender. For example, in many reptiles it is temperature that determines whether an egg becomes male or female. However, in some lizards and alligators, males develop at high temperatures, while in some turtles it is the other way around. In other turtles and crocodiles, males develop only at intermediate temperatures. In the European pond turtle, both genetic and environmental factors determine sex. How did this tremendous variety evolve? This question is one in which researchers are still just beginning to make headway, but the results so far are fascinating.

In principle, since equal sex ratios are often expected to be evolutionarily stable, so will mechanisms that tend to result in equal sex ratios. This phenomenon, whereby selection favours genes that lead to particular sex ratios, is known as sex ratio selection (Chapter 1), and is also the mechanism by which sex allocation evolves. Heterogamety, because of Mendelian inheritance, will tend to result in equal sex ratios, hence, should often be selected for. Heterogamety appears to be relatively conserved (it is the exclusive mechanism in both birds and mammals, both groups with a long ancestry).

Because sex determination and sex allocation both evolve through the same broad mechanism of sex ratio selection, they might be expected to evolve in response to the same environmental factors. For example, it is widely supposed that haplodiploidy has been selected for in insects and mites by the presence of a subdivided mating structure (i.e. a proportional gains asymmetry), the same force that selects for variable sex ratios in subdivided populations, by allowing mothers ease of behavioural control over sex allocation. That certainly seems to be the case in many species; for example, Environmental Sex Determination should also be favoured when offspring quality is differentially affected in the two sexes by factors that act during development, the same force that selects for conditional sex expression (Charnov and Bull 1977).

Inheritance asymmetry may also select for changes in the sex-determining mechanism through conflict between genetic elements over the sex ratio.We have already seen that different genetic elements may differ over the preferred sex allocation strategy. For example, sex chromosomes should favour biased sex ratios while autosomes should generally favour equal sex ratios. Similarly, cytoplasmic elements, such as mitochondria, should favour female biased sex ratios, conflicting again with autosomes. What effects do these conflicts have? One immediate effect is that if a force other than the parental autosomes takes control, this can result in a novel mechanism of sex determination. Under cytoplasmic male sterility in plants, sex is determined not by the ancestral mechanism, but by the presence of mitochondrial mutants and restorer genes which determine whether an individual is cosexual or female. In populations of Plantago lanceolata (Figure 5.7) there are three known CMS mutants and three restorer genes known (de Haan et al. 1997). Some populations have become fixed for one particular mutant and restorer, to the extent that hybrids between populations are sterile. This is rather interesting as it suggests that conflict over sex allocation can lead to reproductive isolation, and potentially even to speciation. The reason conflict can have these effects is that there is selection on both parties in the conflict to rapidly evolve countermeasures such that they have the upper hand in the conflict (an arms race). This can lead to rapid genetic differentiation between populations evolving in isolation.

The other interesting effect of conflict between autosomes and mitochron-dria in CMS systems is that it may facilitate the transition from cosexuality to dioecy. Taxonomically,there is an association between the presence of dioecy and gynodioecy (individuals either cosexual or female). Furthermore, some species appear to have evolved towards dioecy as a result of CMS.When some individuals in a population are fully female as a result of CMS, there can be selection of cosexuals to reduce their allocation to female function in order to restore the population equilibrium of equal allocation. In the common

Fig. 5.7 Cytoplasmic male sterility in P. lanceolata. (a) A normal hermaphrodite flower with both stigmas and anthers, and (b) a male-sterile lacking anthers. Photos courtesy of Hans Peter Koelewijn.

thyme, Thymus vulgaris, cosexual individuals in populations with CMS, indeed, bias their allocation towards males in this way (Atlan 1992). Thus, conflicts as a result of inheritance asymmetry can not only cause biased sex allocation, but perhaps also shifts from cosexuality to separate sexes.

Another interesting example of how conflict over sex allocation can cause the evolution of sex-determining systems comes from the common wood-louse, Armadillidium vulgare (Figure 5.8). This has an ancestral system of female heterogamety, termed ZW females, ZZ males. Some females, however, are infected with Wolbachia and produce an excess of females. The Wolbachia actually works by converting ZZ males into females (ZZ + Wo) (Rigaud 1997). Several field populations now lack the normal W chromosome entirely and consist of ZZ males and ZZ + Wo females. At this stage, these populations have evolved from a normal chromosomal sex-determining system to a cytoplasmic one. A further feminizing factor was then discovered, labelled f, in a population derived from a single ZW female formerly inoculated with Wolbachia. However, it emerged after failure of Wolbachia transmission, suggesting that it might be an autosomal countermeasure to Wolbachia. f can also sometimes become fixed on a male chromosome (Z). This effectively then becomes a new female (W*)

Fig. 5.8 Male (right) and female (left) A. vulgare. Photo courtesy of Didier Bouchon.

chromosome. Thus, conflict can lead to restoration of the chromosomal system again (Figure 5.9). The woodlouse system shows how conflict can lead to potentially rapid turnover in sex determination.

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