Figure 4

Two models on the evolution of sex chromosomes of the schistosomatids. A) Short and Grossman's (1981, 1983) Ideograms proposing the evolution of the sex chromosomes to have followed a similar trend to the rest of the genome, arising from a hermaphrodite ancestor. Two homologous chromosomes that contained the genes for sex determination began to diverge as a result of reduced recombination and gave rise to the ancestors of each schistosomatid group independently. B) Based on the mtDNA data and morphology of the chromosomes, a plausible model of sex chromosome evolution can be put forward suggesting that heterochromatinization events are the major factor in the differentiation of the sex chromosomes in the schistosomatids but also that the pattern of morphological differentiation shows the same relationship as the mitochondrial genome data.