Life History and Reproduction

Campeloma decisum
have various truly interesting aspects regarding their reproduction and their life history.  Their reproduction abilities are extremely diverse among different populations of the species.  Some populations need only one year to mature, whereas others may need two (Dillon et al., 2006).  Campeloma decisum are also ovoviviparous, hence the name of the family that they are from, Viviparidae (Dillon et al., 2006).  The reproduction diversity is apparent in the fact that some populations of Campeloma decisum are merely parthenogenic females, and others undergo outcrossing, and yet there are even other populations that seem to do a combination of both forms (Dillon et al., 2006).  The form of reproduction that is most interesting is parthenogenesis

Research done by Steven G. Johnson proves that Campeloma decisum do not need to undergo outcrossing to successfully carry out parthenogenesis (Johnson, 1992).  The origin of parthenogenesis is believed to be fairly recent due to the clonal variability of the sexual lineages (Johnson, 1992).  The sexual Campeloma decisum lines seem to have different geographical ranges in comparison to the parthenogenic lines, with the parthenogens having the larger range (Johnson, 2006).

How is that possible?
Although the parthenogenesis of Campeloma decisum arose spontaneously, Johnson's 1992 research shows that they continue with their parthenogenic lifestyle thanks to the trematode species called Leucochloridiomorpha constantiae.  The metacercariae stage of Leucochloridiomorpha constantiae select against male Campeloma decisum by limiting sperm.  While sexual populations are not infected with the metacercariae, the parthenogenic populations are infected with it in the brood chamber of females.  However, Leucochloridiomorpha constantiae had to be a part of the sexual population before it was a part of the parthenogenic population (since sexual populations developed into parthenogenic populations).  So then how did this institution of Leucochloridiomorpha constantiae into sexual populations lead to parthenogenesis?  The answer is quite interesting.  When Leucochloridiomorpha constantiae infected the sexual population, it caused a sperm limitation which then created a strong selection for females that were able to reproduce parthenogenically.  This was made possible because the seminal receptacle, where the sperm is stored, and the brood chamber, where the egg is stored, of females were right next to each other, making an easy connection for the metacercariae (Johnson, 1992).

As you can see, the reproduction of Campeloma decisum is not only very complex, but also quite diverse among different populations.

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