Geographic variation in diapause induction and mode of diapause inheritance in Tetranychus pueraricola

Abstract:  Diapause induction and photoperiodic response curves were determined for 33 strains of Tetranychus pueraricola derived from kudzu vine at three constant temperatures (15, 18 and 20°C) under a short-day condition (10 : 14 h; light : dark). Females of all but one of the strains entered diapause at all three temperatures with little variation in diapause percentages among the strains. The exception was the southernmost strain, which was found to be a non-diapause (ND) strain. The critical photoperiod gradually decreased towards the south at a rate of about 1 h for each 5 degrees of latitude. The diapause strains (D1 and D2) exhibited 100% diapause, whereas the ND strain exhibited 0% diapause. By crossing these strains, we determined that 'non-diapause' was a dominant character over 'diapause' and the character was controlled by simple Mendelian inheritance. To clarify why the female progeny from the crosses between the D1 and ND strains did not segregate into the diapause and non-diapause phenotypes in a 1:1 ratio in the B₁ generation, round-robin crosses were carried out among the three strains. The results showed that the F₁ generation was reproductively compatible and showed high egg hatchability with a female-biased sex ratio. In the B₁ generation, the crosses between the D1 and ND strains and between the D1 and D2 strains exhibited extremely low egg hatchability and produced mostly female progeny, whereas offspring from the crosses between the D2 and ND strains showed more than 50% hatchability for B₁ eggs and a female-biased sex ratio. Thus, the absence of segregation observed in the crosses between the D1 and ND strains appears to be due to the severe hybrid breakdown that occurred in the B₁ generation.