Evolution in a putatively ancient asexual aphid lineage: Recombination andrapid karyotype change

Ancient asexual lineages are of great potential significance for understand ing the evolutionary biology of sex, but their existence is controversial. In part, this is because claims of ancient asexuality have rested on negati ve evidence-a mere absence of evidence for sexuality in a taxon. M. Meselso n has suggested a method, discussed by Judson and Normark (1996) and by Bir ky (1996), that has the potential to uncover positive evidence of ancient a sexuality. Phylogenetic relationships between alleles and interallelic dive rgences are predicted to be very different in diploid lineages that lack re combination from those in diploid lineages that undergo recombination. I ha ve applied Meselson's method to the putatively ancient asexual aphid tribe Tramini (Homoptera: Aphidoidea: Lachnidae), using the intron-bearing nuclea r protein-coding gene elongation factor 1 alpha (EF-1 alpha). I found heter ozygosities much lower than intraspecific divergences, indicating that some recombination has occurred, but not discriminating between recombination w ithin an asexual lineage (automixis or mitotic recombination) and outcrossi ng sex. Species of Tramini (especially in the genus Trama) typically have h ighly structurally heterozygous karyotypes that appear to be incompatible w ith regular successful meiosis, and have very high levels of karyotype vari ability within species. I found very high levels of karyotype variability w ithin lineages with identical EF-la and mitochondrial (cytochrome oxidase 1 and 2) genotypes, indicating a high rate of karyotype evolution compared t o the rate of nucleotide substitution.