EFFECTS OF PERICENTRIC REARRANGEMENTS ON RECOMBINATION IN GRASSHOPPER CHROMOSOMES

Abstract

H E significance of chromosomal rearrangements of various kinds in the adapT tive polymorphism of many animal and plant species depends to a large extent on their effects on crossing over. In Drosophila species, crossing over does not normally occur in the male, so that we are only concerned with the effects of rearrangements on crossing over in oogenesis. The evidence (MORGAN, BRIDGES and SCKULTZ 1930; STURTEVANT and BEADLE 1936; STEINBERG 1936, 1937; KOMAI and TAKAKU 1940, 1942; STEINBERG and FRASER 1944; DOBZHANSKY and EPLING 1948; SCHULTZ and REDFIELD 1951; CARSON 1953) has been almost entirely genetic, cytological studies being technically too difficult in this material. Among the effects of hetero-zygosity for paracentric inversions in Drosophila the following are well-established; (1) almost complete suppression of effective crossing over within the limits of the inversion, due in part to elimination of crossover strands in the polar body nuclei, (2) suppression or drastic reduction of crossing over outside the inverted region, both proximally and distally, (3) an increase in the amount of crossing over in the other members of the chromosome complement (the "Schultz-Redfield effect"). Most of those who have studied these phenomena seem to consider that the second type of effect extends as far as the ends of the chromosome limbs, but KOMAI and TAKAKU (1940, 1942) have reported an actual increase of crossing over near the distal end of the Drosophila virilis X chromosome when two heterozygous inversions were present in the proximal region. Highly complex systems of chromosomal polymorphism are now known in many species of grasshoppers, and more particularly in those belonging to the North Ameri-can group Trimerotropi (see WHITE 1954a, pp. 148-155 for a general review). The most characteristic type of rearrangement in this group has the effect of converting an acrocentric chromosome into a metacentric element. Such rearrangements are accordingly pericentric; whether they are actually inversions is still uncertain, but appears probable (a detailed study of the pachytene bivalents is needed to settle this point). Rearrangements of this type are not confined to the group Trimerotropi. They occur, for example, in certain races or populations of the Australian grasshoppers Cryptobothrus chrysophorus and Austroicetes pusilla and in a t least four species of the genus Moraba, which belongs to a different family of grasshopper-like insects (the Eumastacidae). As previously pointed out (WHITE 1951) such rearrangements, when heterozygous, almost completely suppress chiasma formation in the segment between the two centromeres of the bivalent, so that hardly any chromatids with duplications and deficiencies are produced a t meiosis, a t any rate in the male. Thus the regions which are structurally heterozygous are genetically isolated and no gene-transfer can occui between them.