Chromosome organisation in the Australian plague locust Chortoicetes terminifera

Abstract

In Chortoicetes terminifera 45 independently-occurring B-chromosomes were tested and 23 distinct banding variants were detected with either G- or C-banding; six types were found more than once. In particular the Type I banding morph was detected 12 times indicating that individuals carrying this type may be under a different regime of selection compared with individuals bearing other types of banding morph; or the Type I may be subjected to a higher rate of meiotic drive in either or both sexes than other types. Also the Type I appeared to be obviously related to four other banding morphs whereas most types were not obviously related to any other banding morphs, but a few were similar in banding pattern to one or two other types. Three types of B-chromosomes were found in three or more different populations. A relatively high frequency of the Type I banding morph was found in one population, which was probably mainly composed of non-migratory individuals, and also in a laboratory-raised population. The most likely mechanisms for small changes in the banding sequence of the B-chromosomes are three-break insertions which are often indistinguishable from inversions. Rearrangements which add or delete bands, or sequences of bands, to or from B-chromosomes are probably the result of exchanges which are now known to take place in rare individuals with two B-chromosomes. The most distal region of all the banding morphs of the B-chromosome in C. terminifera, plus a short interstitial region in some types, is not late-replicating and has the banding characteristics of euchromatin. The rest of the chromatin of the B-chromosomes is heterochromatic and is the latest replicating heterochromatin in the whole genome. It consists of G-bands, which are also deeply stained with C-banding, and alternating G-interbands, which in turn are stained “grey” with C-banding. Both of these staining combinations are seen in heterochromatin of the normal complement. The heterochromatin of the B-chromosomes is condensed throughout 1st meiotic prophase in the male and in all somatic interphase nuclei where it can be quickly detected using the G-banding technique. The B-chromosome has a relatively constant, acrocentric morphology with a tendency to increase of length of the long arm as band numbers increase. Isochromosomes of the long arm have been seen only in laboratoryraised embryos. From egg pods with significantly fewer than expected B-chromosomes it is strongly suggested that more than one male may fertilize the eggs in a single pod.