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

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Abstract
Synopsis: A 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 andM. 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 jenkinsiandCampeloma rufumare also of the ameiotic type but have a single maturation division. No males were ever found in the diploid race ofM. tuberculatusand inM. scabra.In the polyploid race ofM. tuberculatusand inM. lineatussporadic 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 ofM. tuberculatusand one inM. lineatusalthough 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.