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

Abstract

Ancient asexual lineages are of great potential significance for understanding the evolutionary biology of sex, but their existence is controversial. In part, this is because claims of ancient asexuality have rested on negative evidence-a mere absence of evidence for sexuality in a taxon. M. Meselson has suggested a method, discussed by Judson and Normark (1996) and by Birky (1996), that has the potential to uncover positive evidence of ancient asexuality. Phylogenetic relationships between alleles and interallelic divergences are predicted to be very different in diploid lineages that lack recombination from those in diploid lineages that undergo recombination. I have applied Meselson's method to the putatively ancient asexual aphid tribe Tramini (Homoptera: Aphidoidea: Lachnidae), using the intron-bearing nuclear protein-coding gene elongation factor 1α (EF-1α). I found heterozygosities much lower than intraspecific divergences, indicating that some recombination has occurred, but not discriminating between recombination within an asexual lineage (automixis or mitotic recombination) and outcrossing sex. Species of Tramini (especially in the genus Trama) typically have highly structurally heterozygous karyotypes that appear to be incompatible with regular successful meiosis, and have very high levels of karyotype variability within species. I found very high levels of karyotype variability within lineages with identical EF-1α and mitochondrial (cytochrome oxidase 1 and 2) genotypes, indicating a high rate of karyotype evolution compared to the rate of nucleotide substitution.