XVI.—Cytological Studies of Melaniidæ (Mollusca) with Special Reference to Parthenogenesis and Polyploidy. I. Oögenesis of the Parthenogenetic Species ofMelanoides(Prosobranchia-Gastropoda)

Abstract

SynopsisA Study of the oögenesis and maturation in four species of melaniid snails, viz. Melanoides tuberculatus—diploid race 2n = 32, M. tuberculatus—polyploid race with 90-94 chromosomes, M. lineatus—polyploid with 71-73 chromosomes and M. scabra—polyploid with 76-78 chromosomes, showed these species to be thelytokous. These are instances of ameiotic parthenogenesis with two equational maturation divisions and are recorded for the first time in molluscs, the only other known instance in the animal kingdom being shown by the cockroach, Pycnoscelus surinamensis. The other parthenogenetic molluscs known so far, viz. Potamopyrgus jenkinsi and Campeloma rufum are also of the ameiotic type but have a single maturation division. No males were ever found in the diploid race of M. tuberculatus and in M. scabra. In the polyploid race of M. tuberculatus and in M. lineatus sporadic males occur, forming about 3 per cent of the population in the former and 0·01 per cent in the latter. Further evidence for ameiotic parthenogenesis is provided by the complete absence of sperms in the receptaculum seminis and brood pouch and also by the absence of sperm elements or any fusion of nuclei in the egg. The chromosome numbers in the different species have been determined from maturation metaphase and have been checked from first cleavage metaphase. Polyploidy is associated with parthenogenesis in three of these species.Oögonial cell divisions are generally absent. Only three oögonial metaphase plates were seen in the diploid race of M. tuberculatus and one in M. lineatus although a very large number of preparations of the ovary were examined. The author holds the view that the maturation prophase is the continuation of the oögonial early metaphase stage without intervening metaphase, anaphase and telophase. The prochromosomes seen in proleptotene stage are already split and the succeeding stages are characterized by the absence of zygotene, pachytene or any pairing and chiasma formation. Each split chromosome behaves functionally like a bivalent throughout maturation prophase and separates into daughter chromosomes at the first division. The second division is equational as in normal maturation.

Density and variability of dinucleotide microsatellites in the parthenogenetic polyploid snail Melanoides tuberculata

“…The multiple banding patterns observed at each locus is consistent with polyploidy in M. tuberculata (Jacob 1959). The reproducibility of these patterns is secured by: (i) repeated PCR assays on the same individuals; (ii) the fact that many individuals of a given morph have the same genotype whatever the site and year of sampling; and (iii) the fidelity of genotype transmission across generations.…”

“…This is an expected consequence of variable ploidy levels, as well as of the origin (auto‐ vs. allopolyploidy) and age of polyploidy. The origin of polyploidy in M. tuberculata indeed involves complex and repeated allo‐ and autopolyploidization events (Jacob 1959). Allopolyploidization through hybridization may have associated more or less homologous sets of chromosomes.…”

“…It exhibits a wide, discrete, morphological variation (Pointier 1989). The creation of morphs involved complex events of polyploidization, and morphs therefore are generally more than tetraploid (Jacob 1959). However, whether this variation (morphs) corresponds to genetic clones, or whether each morph is a composite of many clones, has still not been determined.…”

Neritid and thiarid gastropods from French Polynesian streams: how reproduction (sexual, parthenogenetic) and dispersal (active, passive) affect population structure

“…Electrophoretic studies of individuals from three populations there indicated a high level of polymorphism in each population (Livshits & Fishelson, 1983). A low percentage of males has also been recorded in other populations (Jacob, 1957). Dybdahl & Lively (1995) suggested that the many clonal populations they found in Potamopyrgus antipodarum were derived from sympatric sexual populations.…”

Punctuated equilibrium, morphological stasis and the palaeontological documentation of speciation: a reply to Fryer, Greenwood and Peake's critique of the Turkana Basin mollusc sequence

“…This species is arguably one of the most eurytopic freshwater organisms known; although it occurs almost ubiquitously in Africa with other common freshwater molluscs in 'normal' river and lake waters, it has an astonishing resistance to extreme water chemistries (e.g. Jacob, 1957;Samaan & Aleem, 1972;Tchernov, 1973), extremely high water temperatures (e.g. Raymond, 1852), and high suspended silt loads and flocculent substrates (e.g.…”