Proteomic analysis of F1 hybrids and intermediate variants in a littorina saxatilis hybrid zone

Proteomic analysis was carried out on the Crab (upper-shore) and Wave (lower-shore) ecotypes of Littorina saxatilis from a hybrid zone at Silleiro Cape, Spain. Proteome profiles of individual snails were obtained. Protein expression in F1 hybrid snails bred in the laboratory and snails with intermediate shell phenotypes collected from the mid-shore were compared with Crab and Wave ecotypes using analytical approaches used to study dominance. Multivariate analysis over many protein spots showed that the F1 snails are distinct from both ecotypes but closer to the Wave ecotype. The intermediate snails are highly variable, some closer to the Crab and others to the Wave ecotype. Considered on a protein by protein basis, some proteins are significantly closer in expression to the Crab and others to the Wave ecotype for both F1 and intermediate snails. Furthermore, a significant majority of proteins were closer in expression to the Wave ecotype for the F1, consistent with the multivariate analysis. No such significant majority towards either the Crab or Wave ecotype was observed for the intermediate snails. The closer similarity of F1 and Wave ecotype expression patterns could be the result of similar selective pressures in the similar mid-shore and low-shore environments. For a significantly larger number of proteins intermediate snails were closer in expression to the ecotype having the lower expression, for both Crab and Wave ecotypes. This is somewhat unexpected as lower expression might be expected to be an indication of impairment of function and lower fitness. Proteomic analysis could be important for the identification of candidate proteins useful for gaining improved understanding of adaptation and barriers to gene flow in hybrid zones.

Comparing shape along growth trajectories in two marine snail ecotypes of Littorina saxatilis: a test of evolution by paedomorphosis

ABSTRACT Two sympatric ecotypes (‘crab’ and ‘wave’) of Littorina saxatilis are adapted to different microhabitats. It has been claimed, based on the comparison of proteomic differentiation across ontogeny, that the wave ecotype may have evolved by paedomorphosis from an ancestor more similar to the crab ecotype. Here, we test the paedomorphosis hypothesis at a morphological level by comparing crab and wave specimens from two localities using the pattern of shell shape differentiation across ontogeny. The results obtained show a pattern of shell shape differentiation similar to the one observed in previous proteomic studies, but such a pattern could be caused by different modes of evolution, and not necessarily by paedomorphosis. This study emphasizes that in addition to studying the pattern of differentiation, the direction of the evolutionary change across ontogeny has to be analysed before conclusions can be drawn on particular developmental modes of evolution.

New Data on Spermatogenic Cyst Formation and Cellular Composition of the Testis in a Marine Gastropod, Littorina saxatilis

Knowledge of the testis structure is important for gastropod taxonomy and phylogeny, particularly for the comparative analysis of sympatric Littorina species. Observing fresh tissue and squashing fixed tissue with gradually increasing pressure, we have recently described a peculiar type of cystic spermatogenesis, rare in mollusks. It has not been documented in most mollusks until now. The testis of adult males consists of numerous lobules filled with multicellular cysts containing germline cells at different stages of differentiation. Each cyst is formed by one cyst cell of somatic origin. Here, we provide evidence for the existence of two ways of cyst formation in Littorina saxatilis. One of them begins with a goniablast cyst formation; it somewhat resembles cyst formation in Drosophila testes. The second way begins with capture of a free spermatogonium by the polyploid cyst cell which is capable to move along the gonad tissues. This way of cyst formation has not been described previously. Our data expand the understanding of the diversity of spermatogenesis types in invertebrates.

A developmentally descriptive method for quantifying shape in gastropod shells

The growth of snail shells can be described by simple mathematical rules. Variation in a few parameters can explain much of the diversity of shell shapes seen in nature. However, empirical studies of gastropod shell shape variation typically use geometric morphometric approaches, which do not capture this growth pattern. We have developed a way to infer a set of developmentally descriptive shape parameters based on three-dimensional logarithmic helicospiral growth and using landmarks from two-dimensional shell images as input. We demonstrate the utility of this approach, and compare it to the geometric morphometric approach, using a large set of Littorina saxatilis shells in which locally adapted populations differ in shape. Our method can be modified easily to make it applicable to a wide range of shell forms, which would allow for investigations of the similarities and differences between and within many different species of gastropods.

Female size and pre-copulatory sexual selection by males of intertidal snail (Littorina saxatilis)

Littorina saxatilis, the rough periwinkle, is highly promiscuous and polyandrous; female L. saxatilis snails rarely reject male advances. Sexual selection by males has been observed in this species. Sexual selection is one of the driving forces of evolution and may be a significant contributor to partial reproductive isolation between two ecotypes of L. saxatilis. Male sexual selection based on female size was investigated for wave-exposed crab ecotype snails in this study. Males and females were measured and placed in mating arenas. Males were presented with a choice of a smaller or larger female relative to themselves. Mating frequency and duration was recorded. A total of 19 successful trials were performed. A significantly higher mating frequency (p<0.005) was associated with larger females, indicating that males exhibit strong pre-copulatory sexual selection. Mating duration was significantly longer for larger female mating events (p<0.05) but was also equally determined by male size (p<0.05) as larger males tended to mate for longer. These results indicate that a strong sexual selection by males exists for L. saxatilis, in which larger females are preferred. This behavior may be a response to perceived fecundity of females, and/or may be a product of selection against hybrids.