The evolutionary process leading to the emergence of viviparity in Squamata consists of lengthening the period of egg retention in utero coupled with marked reduction in the thickness of the eggshell. We used light microscopy and scanning electron microscopy to study uterine structure during the reproductive cycle of oviparous and viviparous females of the reproductively bimodal Lacerta vivipara. We compared the structure of the uterine shell glands, which secrete components of the eggshell, during preovulatory and early gestation phases of the reproductive cycle and also compared histochemistry of the eggshells. The uterine glands of both reproductive forms undergo considerable growth within a period of a few weeks during folliculogenesis and vitellogenesis preceding ovulation. The majority of the proteinaceous fibers of the shell membrane are secreted early in embryonic development and the uterine glands regress shortly thereafter. This supports previous observations indicating that, in Squamata, secretion of the shell membrane occurs very rapidly after ovulation. The most striking differences between reproductive modes were larger uterine glands at late vitellogenesis in oviparous females, 101 microm compared to 60 microm in viviparous females, and greater thickness of the shell membrane during early gestation in oviparous females (52-73 microm) compared to viviparous females (4-8 microm). Our intraspecific comparison supports the conclusions of previous studies that, prior to ovulation, the uterine glandular layer is less developed in viviparous than in oviparous species, and that this is the main factor accounting for differences in the thickness of the shell membrane of the two reproductive forms of squamates.
Intraspecific phylogeographic methods provide a means of examining the history of genetic exchange among populations. As part of a study of the history of Helix aspersa in the Western Mediterranean, we performed a phylogenetic analysis based on partial sequences of the mitochondrial large ribosomal subunit (16S) gene. Our samples include 31 H. a. aspersa populations from North Africa previously investigated for anatomical and biochemical characters. To clarify subspecific relationships, three individuals of the subspecies H. a. maxima were also studied. The molecular phylogeny inferred agrees largely with previous results, in splitting H. a. aspersa haplotypes into an eastern and a western group. H. a. maxima haplotypes form a third lineage arising before the H. a. aspersa groups. Divergence times estimated between the lineages suggest that dispersal during Pleistocene glaciation and vicariance events due to Pliocene geological changes in the western Mediterranean may both have played a significant part in the establishment of the present range of H. aspersa.
Local patterns of genetic variation were analysed in the land snail Helix aspersa for 32 populations sampled within a patchy agricultural landscape: the polders of the Bay of Mont-Saint-Michel (France). This investigation examined the allele frequencies at four enzymatic markers and five microsatellite loci through the genotyping of 580 individuals. A strongly significant population genetic substructuring (mean F ST ¼ 0.088, Po0.001) was found at the scale of the whole polders area (3050 ha) and both categories of markers displayed a similar magnitude of spatial genetic differentiation. We did not find any obvious effects of habitat fragmentation on the distribution of genetic variability. Despite the reality of habitat patchiness and environmental instability (related to farming practices), an isolation by distance process was clearly depicted, although selective pressures cannot be ruled out for one enzymatic locus. Overall, genetic drift, along with occasional long-distance episodes of gene flow, was presumably the most likely evolutionary force that shaped the observed pattern of genetic variation.
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