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Zoological Science
Abstract
We examined whether the ploidy level of eggs from sexual and parthenogenetic females of the weevil Scepticus insularis changes when field-collected, egg-bearing females are exposed to low temperature, as suggested by a previous study. We observed no change in ploidy level in eggs laid by sexual females (n = 15) treated by low temperature (1.1–6.7°C). In contrast, eggs laid by parthenogenetic females were not stable in ploidy level, as 11 of 16 females tested laid both diploid and triploid eggs even before the low-temperature treatment. After the low-temperature treatment, the proportion of triploid eggs to total eggs increased in nine individuals and decreased in the rest, and the effect of the treatment on the overall change in frequency was significant. Our results thus show that exposure to cold does not induce a change in egg ploidy in the sexual form of S. insularis, although cold may affect ploidy levels in the eggs of parthenogens. Additionally, eggs laid by laboratory-reared, virgin sexual females (n = 13) did not hatch after the low-temperature treatment, indicating that the treatment did not induce parthenogenetic reproduction in normally sexually reproducing females of S. insularis. We also examined the effect of low temperature on the ploidy level of eggs from parthenogenetic females (n = 4) of another weevil species, Catapionus gracilicornis, and confirmed that the proportion of triploid eggs steeply decreased and that of diploid eggs increased after exposure to cold, being consistent with those of previous studies.
We thank H. Watabe and G. Takaku for help during fieldwork; Y. Matsuda and N. Kitamura for methodological advice; N. Kasuya for assistance in insect rearing; and M.H. Dick for comments on the manuscript. This study was supported by the 21st Century Center of Excellence (COE) Program “Neo-Science of Natural History” at Hokkaido University (Program Leader: H. Okada) financed by the Ministry of Education, Culture, Sports, Science and Technology, Japan, and by a Grant-in-Aid to S. Akimoto (23370037) from the Japan Society for the Promotion of Science. Part of this paper derives from a PhD dissertation (Tada, 2012) submitted to the Graduate School of Science, Hokkaido University, Sapporo.
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