A UNIQUE TYPE OF SEX CHROMOSOME MECHANISM IN AN AUSTRALIAN MANTID

Abstract

Two kinds of sex chromosome mechanism are well known in mantids (see Hughes Schrader, 1950, 1953, for review of earlier literature). In 23 genera an XO (, ) mechanism has been found, the X being a metacentric in all species thus far examined. This clearly represents the primitive condition for the suborder as a whole. In 15 other genera an X1X2Y ( 6 ): X1X1X2X2 ( 9 ) system exists, a characteristic sex chromosome trivalent being formed at meiosis in the male, in which the left arm of X1 and the right arm of X2 are paired, respectively, with the two limbs of the Y. Several Australian genera may now be added to that total (White, unpublished). As first proposed by White (1940, 1941 ), and accepted in principle by Hughes Schrader (1950) and Callan and Jacobs (1957), it seems probable that the X1X2Y genera represent a strictly monophyletic group which arose from a single species in which the XO mechanism gave rise to the X1X2Y one by a mutual translocation between the X and a metacentric autosome. Oguma (1946) has proposed that the two groups of genera should be formally recognized as Monosomata (with a single sex chromosome in the male) and Trisomata (with three sex chromosomes). The chromosome numbers (2n d ) are known to range from 15 to 39 in the Monosomata, but the great majority of the Trisomata have 2n d = 27, which was almost certainly the primitive number in this group. The only exceptions are Choeradodis rhombicollis with 2n 8 = 31 (Hughes Schrader, 1943), Algerian material of Sphodromantis