The recurrence of chromosome fusion in inter-population hybrids of the grasshopper Atractomorpha similis

Abstract

Single-pair matings of the grasshopper Atractomorpha similis (2n=19 ♂; 20 ♀) were set up in the laboratory. Among the progeny, three families out of eleven exhibited a tendency toward chromosome fusion. From eight to ten different fusions were observed involving one interstitial and 15 to 19 procentric exchange events. At least seven of the fusions were not transmitted from the parent generation but arose spontaneously within the individuals in which they were observed. In one family, more than one fusion type arose within each of two individuals. It is concluded that the three families with the fusion syndrome have a transmissible tendency towards a particular type of chromosome rearrangement, namely centric fusion. The fusions observed involve at least six of the ten chromosome pairs. Thus the tendency to procentric exchange is non-specific in respect of the linkage groups involved. This phenomenon is discussed in relation to the process of karyotypic orthoselection and the data presented are used to model this process. The relatively low frequency of chromosome rearrangement in natural populations of this species suggests that the high frequency observed in laboratory populations is a consequence of crossing geographically distant populations which would not normally interbreed.