Chromosomal Evolution of Delena cancerides
Abstract
The huntsman spider Delena cancerides is known to be highly chromosomally variable, and to have reduced recombination near fusions. However, this species has previously only been interpreted with reference to the classical models of chromosomal speciation, the expectations of which it does not fit well. Broad-scale sampling of this spider has revealed extensive chromosomal diversity and complexity. Twenty one chromosomally differentiated populations (karyomorphs) of this spider have now been described, including those with the putatively ancestral configuration of all telocentric bivalents at meiosis (tII), and many that are saturated for Robertsonian fusions. These include up to six different karyomorphs with metacentric bivalents (mII), eight karyomorphs that form a chain of chromosomes at male meiosis, and six karyomorphs that form two separate but co-segregating chains.¶ A computer simulation was used to test hypotheses regarding the evolution of this chromosomal diversity, which indicated that fusions are likely to have accumulated gradually, possibly due to meiotic drive. Historical phylogeographic analyses have shown that deep cryptic divisions exist which are concordant with the chromosomal diversity.¶ Hybridization experiments have suggested that many hybrid zones between karyomorphs of this species are tension zones, and that genetic incompatibilities are likely to play an important role in generating partial reproductive isolation of karyomorphs. Furthermore, several hybrid zones appear to have been modified by staggered clines. The staggering of clines is thought to ameliorate reproductive isolation mechanisms that are dependent on epistatic fitness interactions, and so may prevent diverging populations progressing towards speciation. ¶Therefore, on the basis of the available evidence, D. cancerides may fit the recombination suppression model of chromosomal speciation, although it may be unlikely that the karyomorphs will progress towards full species status. Hence, this species may in the future make a highly informative model organism for investigating the early stages of genetic reproductive isolation associated with chromosomal rearrangements.