Inheritance and meiotic behaviour of a de novo chromosome fusion in the aphid Myzus persicae (Sulzer)

A de novo tandem fusion between autosomes 2 and 3 (A2+3), arising in the co urse of laboratory crosses of sexual morphs of two clones of the aphid Myzu s persicae, was stable through more than 180 generations of parthenogenetic (clonal) reproduction. Studies of its inheritance through the sexual phase , and segregation from an amplified esterase marker gene, showed that cross ing over occurred during oogenesis, but in in spermatogenesis, confirming p revious cytological observations. Only a small number of progeny resulted f rom attempts at selfing fusion heterozygotes, and none of these was homozyg ous for the fusion. A2+3 paired in parallel alignment with the separate A2 and A3 to form a trivalent at prophase I of spermatogenesis. Fusion heteroz ygotes had a segregation problem at anaphase I of meiosis, A2+3 forming a c hromatin bridge between the daughter spermatocytes in about 42% of dividing cells, which could be attributed to alternate orientation in the trivalent (A2 and A3 paired with opposite sides of A2+3) in the preceding metaphase I. Males heterozygous for an A2 dissociation were also studied and found to have much less of a segregation problem, despite showing similar orientati on patterns at metaphase I. Possible reasons for this difference and the si gnificance of the findings in relation to karyotype evolution in aphids are discussed.