Analysis of the phenotypic effects of B chromosomes in a natural population of Metagagrella tenuipes (Arachnida: Opiliones)

Abstract

Phenotypic effects of B chromosomes in a natural population of Metagagrella tenuipes (Arachnida: Opiliones) were studied. Mean number of Bs per individual in the population studied was 6.0, and remained stable during two successive summers of 1997 and 1998. In contrast to the number of B chromosomes, ratios between individuals possessing odd and those possessing even numbers of Bs changed during both collection seasons: the proportion of harvestmen with an even number of Bs decreased from June-July to October-November. A possible reason for this may be a difference in susceptibility to parasites between B-odd and B-even harvestmen. In the group of B-even individuals the percentage of infected harvestmen in the June-July samples was much higher compared to the B-odd group. In addition, the infection rate in the B-even group decreased more sharply than among B-odd harvestmen. In the group of B-even harvestmen infection was associated with reduced body size, whereas no such association was found among B-odd harvestmen. In the group of B-even individuals there was a U-shaped relationship between number of Bs and the probability of being infected by parasites, and an inverted-U-shaped relationship between body size and number of Bs. No such associations were found in the group of B-odd harvestmen. Seasonal selection is suggested to be a main factor contributing to the B-chromosome polymorphism in M. tenuipes.

Geographic and seasonal variations of the number of B chromosomes and external morphology in <i>Psathyropus tenuipes</i> (Arachnida: Opiliones)

“…3 together with data obtained in 1997 and 1998 at the same population (Gorlov and Tsurusaki, 2000a). In 1994, the number of Bs increased from August (2.7) to October (6.0).…”

“…0.0001). It is unlikely that the number of Bs differed between sexes in this generation, since it has been shown that the number of Bs does not differ between sexes, at least in subadults and younger adults just after the final molting (Tsurusaki, 1993;Gorlov and Tsurusaki, 2000a). Decrease of the number of Bs in embryos was also ascertained in additional rearing experiments using females from the same population in 1995 (Tables 1 and 2).…”

“…3, middle and bottom) as already reported by Gorlov and Tsurusaki (2000a, b). Gorlov and Tsurusaki (2000a) found an interesting trend that the higher frequency of males with even modal number of Bs in June-July decreased with time in 1997. However, no such trend was observed in the 1994 samples.…”

“…1), though some of them carry euchromatic segments located on terminal regions or rarely in the middle of the chromosome; (5) Bs behave as univalents at meiosis (Fig. 1); (6) Presence of a peculiar odd-even effect, suggesting differences in susceptibility to parasites between B-odd and B-even harvestmen (Gorlov and Tsurusaki, 2000a; but see discussion section).…”

“…All the specimens are males except for eggs. Gorlov and Tsurusaki (2000a). A line in the body of the box and the ends of the box locate median and the 25th and 75th quantiles, respectively.…”