Aphids have a sex determination model based on the presence of two X chromosomes (XX) in females and a single X chromosome (XO) in males. Previous studies suggested that X chromosome loss during male determination was random and that both X chromosomes have the same chances to be inherited in males.
On the contrary some colleagues and me suggested in a paper on Comparative Cytogenetics that the presence of strong biases in the transmission of sex chromosomes so that X chromosomes may have a non-mendelian inheritance in aphids. On the basis of the results obtained in potato aphid Macrosiphum euphorbiae, we suggest the presence of a non-random elimination of one X chromosome during male determination process. In particular the unequal distribution of rDNA genes between the two X chromosomes could favour the loss of the X chromosome with few rDNA genes. This result is due to the fact that all the aphid eggs during the prophase present two X chromosomes linked by rDNA genes. However, in eggs developing as females, the connection is quickly lost, but in male generating eggs the X chromosomes remain attached by rDNA genes and undergo a sort of non-canonical reductional division so that at the end of this peculiar division the egg has one X chromosome only and it is determined as a male. X chromosomes with few rDNA genes seem to be less sticky that X chromosomes with larger rDNA regions so that their inheritance is not random and X chromosomes with few rDNA genes may be lost and not inherited in males.
Mutations in the chromosome structure could therefore affect chromosomal inheritance even if these changes do not affect regions involved in centromere functioning.
Reference:
Monti, V., Manicardi, G.C. Mandrioli, M. (2011). Cytogenetic and molecular analysis of the holocentric chromosomes of the potato aphid Macrosiphum euphorbiae. Comparative Cytogenetics 5:163-172.
