Figure 1.
The karyotypes occurring in the hybrid zone.
The karyotypes are organised by sex and the generation they first appear (in a cross between neo-XY and XO populations). The lower pink chromosome is the ancestral X. The upper chromosome can be an unfused Au (yellow), or fused to the X (as part of the neo-X, also shown in yellow), or a Y-chromosome (green). Each karyotype has been labelled by a letter, for reference. The expected frequency of each karyotype at equilibrium is shown for the neutral case after a cross between equal numbers of new-XY and XO. Note the 1∶4 ratio of the Y to unfused Au in the parental populations (made of A, B, F, G).
Figure 2.
The karyotypes produced by each possible mating.
The letters refer to the karyotypes in Figure 1. The area of each cell indicates the relative proportion of each karyotype in a mating. Offspring with the same karyotypes as their parents are indicated in bold. This matrix forms the basis of the computer simulations.
Figure 3.
The fate of sexually antagonistic alleles in a single population.
We consider an ancestral autosomal locus, which had two sexually antagonistic alleles: a was favoured in males and b in females. 3A. The outcome of selection on the b allele in the ancestral population as a function of the fitnesses of the two homozygotes. The central area of fitness combinations results in a stable polymorphism (delineated by the contours pa = 0.001 and pa = 0.999). In this example the alleles were additive. 3B. The evolutionary dynamics change if one of the b-bearing autosomes fuses to the X-chromosome. We illustrate this effect by plotting the frequency of the b-neo-X haplotype 1 000 generations after it has been introduced at low frequency (0.04). The neo-X spreads for some fitness combinations (the central area enclosed by 0.001 and 0.999 contours). The spread of the neo-X was opposed by weak selection against females heterozygous for the fusion (genotypes H and I from Figure 1 were assumed to suffer a 1% reduction in fertility). Note, that mild sexually antagonistic selection (i.e. the region near the point (1,1)) is insufficient to favour the fused X.
Table 1.
The finesses of the karyotypes under different forms of selection.
Figure 4.
Summary of the simulation results for the Y and fusion clines.
The selection is described according to the genotype with reduced fitness (w♂(AU·), w♀(Y·), w♀(FU)) as set out in the first row of Table 1. In the particular examples illustrated here the fitness was reduced by setting the selection coefficent (sm, or sf) to 0.1 (with dm = df = 1). The only exceptions are in the upper two panels, in which case the extreme selection coefficient was set to 0.9.
Table 2.
The speed of the fusion cline's advance.