Chromosome inheritance? Not the same for all the chromosomes

Aphids are insects with 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 probability to be inherited in males. On the contrary, some authors suggested 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 the male determination process. In particular, the unequal distribution of ribosomal DNA (rDNA) genes between the two X chromosomes could favour the loss of the X chromosome with fewer 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.