High diversity of structurally heterozygous karyotypes and rDNA arrays in parthenogenetic aphids of the genus Trama (Aphididae: Lachninae).

Karyotypes of permanently parthenogenetic aphids of three species of the genus Trama show great diversity, particularly in the number and distribution of chromosomal elements containing highly repetitive sequences. Sampling at only a few sites in southern England, chromosome number varied from 14 to 23 in T. troglodytes, 9-12 in T. caudata and 10-14 in T. maritima, with some colonies having individuals of more than one karyotype. This variation was paralleled by differences in the number and distribution of rDNA arrays revealed by in situ hybridization. This high intraspecific karyotype diversity contrasts with very low genetic diversity in the same populations, suggesting rapid karyotype evolution. Although T. troglodytes feeds on many species of composite plants there was no evidence of any karyotype-associated host race formation.