Does size influence
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Norton and Bronson2006.pdf | |
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Norton et al. 2008.pdf | |
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Norton and Newman 2016.pdf | |
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Albinism is a Mendelian Trait
In many species, the difference between pigmented and non-pigmented individuals is due to a single recessive Mendelian gene. We have demonstrated that this is also the case in the freshwater hermaphroditic snail, Planorbella (Helisoma) trivolvis, and have used this difference in pigmentation as a genetic marker of parentage to better understand the reproductive biology of these animals. We carried out crosses between albino (A) individuals from an inbred laboratory strain and pigmented (P) individuals one generation removed from a natural population, assessed pigmentation of progeny, and measured egg production for three weeks. Results of parental, F1, and F2 generations and backcrosses to pure-breeding albinos were consistent with a Mendelian single gene inheritance pattern. Norton, CG, AF Johnson, BM Nelson. 2018.a The genetic basis of albinism in the hermaphroditic freshwater snail Planorbella trivolvis. American Malacological Bulletin 36: 153-158. norton_et_al_2018a.pdf Maternal and Paternal influences on Fecundity Components In the first generation crosses (AxA, AxP and PxP), albino individuals laid significantly more egg masses than did pigmented individuals, regardless of paternal contribution (A or P), indicating a maternal influence on egg mass production. Conversely, there was a paternal influence on the number of eggs per mass with pigmented sperm donors contributing higher average numbers of eggs to egg masses. These results indicate that the number of egg masses laid is determined in part by a maternal contribution, and that the number of eggs laid in each mass may be limited by the availability of sperm. Because we never observed albino progeny from any of the AxP reciprocal crosses, we also confirmed that selfing is rare in H. trivolvis. Norton, CG, AF Johnson, BM Nelson. 2018.b Population differences in fecundity components in the hermaphroditic freshwater snail Planorbella trivolvis. American Malacological Bulletin 36: 23-31. norton_et_al_2018b.pdf |
First Sperm Precedence We used pure breeding albino and pigmented descendants of the F2 crosses to determine the nature of sperm precedence in this species. Because P. trivolvis store sperm, we predicted that sperm from the first mate would fill the spermatheca, resulting in low second sperm success. We isolated albino snails until maturity and mated half first with a pigmented snail and then an albino snail and the other half in the reverse order. We collected egg masses 1, 3, and 5 weeks after mating and examined embryos under an inverted microscope at 40x for the presence or lack of eye pigmentation to identify paternity. Approximately 85% of eggs were fertilized by the first partner, indicating strong first sperm precedence. In addition, there was significantly lower first sperm precedence when the first mate was albino, suggesting differences in sperm transfer or competitive ability between albino and pigmented individuals or a preference for pigmented partners. Norton, C.G. and M.K. Wright. 2019. Strong first sperm precedence in the freshwater hermaphroditic snail Planorbella trivolvis. Invertebrate Reproduction & Development doi.org/10.1080/07924259.2019.1630019 norton_and_wright_2019.pdf |