Origin of chromosomal rearrangement: Phylogenetic relationship between X0/XX and XY/XX chromosomal races in the brachypterous grasshopper Podisma sapporensis (Orthoptera: Acrididae)

The brachypterous grasshopper Podisma sapporensis Shiraki (Orthoptera: Acrididae) consists of two major chromosomal races with different sex chromosome systems. In the XO/XX race, the diploid number of chromosomes is 2n = 23 (XO) in males and 2n = 24 (XX) in females. In the XY/XX race, the diploid number is 2n = 22 (XY) in males and 2n = 22 (XX) in females, owing to Robertsonian fusion between an autosome and the X chromosome. The XO/XX and XY/XX races are allopatrically distributed, and each race contains geographical populations characterized by different chromosomal inversions. A previous hypothesis suggested that the XY/XX race was derived from the XO/XX race. To test this hypothesis, we examined mitochondrial DNA (mtDNA) sequence variation in two regions (cytochrome oxidase subunit II and 16S ribosomal DNA) among 29 P. sapporensis individuals representing five XO/XX and 12 XY/XX populations. The maximum parsimony tree of mtDNA indicated that neither of the two chromosomal races was monophyletic. Northern populations of XY/XX race did not join a clade consisting of more southern XY/XX populations but were included in a clade consisting of XO/XX populations. On the basis of these results, we propose two hypotheses for the differentiation between the northern and southern XY/XX populations. First, the XY/XX karyotype may have occurred more than once. Second, introgression of mtDNA may have occurred across adjacent populations possessing different karyotypes.