Abstract
Autosomal fusion in spiders is generally an all-or-nothing phenomenon. That is, in respect of the autosomes, spider karyotypes are usually totally telocentric or saturated for fusions.D. cancerides behaves in this manner, but possesses a number of racial cytotypes with different fusion combinations. Two of these are heterozygous for a number of fusions which include a sex chromosome and consequently form long chain multiples during male meiosis. Data are presented to support the hypothesis that these races arose as the result of hybridisation between parental forms homozygous for different fusion combinations. A generalisation of this model states that, when an X-autosome fusion is present, hybridisation of two races with monobrachial homology may result in either of two distinct outcomes. As has been discussed elsewhere, selection may result in premating isolation and hence speciation, however a plausible alternative is that selection for routine alternate segregation will produce stable sex-linked translocation heterozygosity. The adoption of either strategy negates selection for the other and so an evolutionary race between the two ensues.
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