Abstract
The results presented indicate that the sex determination mechanism in the Texas race ofR. hastatulus ♀ 2n = 10 (XX + 8A); ♂ 2n = 10 (XX + 8A)] is intermediate between theX/Y andX/A systems. In this race, sex is determined to some extent by theX/A balance, but theY chromosome also affects sex expression, maleness or intersexuality being correlated with different ratios ofX andY chromosomes.The results obtained for the Texas race are fully compatible with data presented by Smith (1963) for the North Carolina race [♀ 2n = 8 (XX + 6A); ♂ 2n = 9 (XX 1 Y 2 + 6A)]. It may be concluded that evolution of the karyotype in this species is not accompanied by changes in the mechanism of sex determination.
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