The chromosome cycle in the land snail Catinella vermeta (Stylommatophora, Succineidae)

TitleThe chromosome cycle in the land snail Catinella vermeta (Stylommatophora, Succineidae)
Publication TypeJournal Article
Year of Publication1966
AuthorsPatterson C.M, Burch JB

At least 3 members of the genus Catinella are unique among the Stylommatophora because of their low chromosome numbers (C. rotundata of Hawaii, n=5 (2n=10); C. vermeta and C. texana n=6 (2n=12)). Because of this low number and relatively large size of the chromosomes, C. vermeta is particularly well suited for a study of the chromosome cycle during spermatogenesis. The cycle does not differ, in general, from that found in other animals, and especially resembles the cycle we have observed in other euthyneuran snails. Details, however, are much clearer, the various stages more easily recognized, and those not detected with certainty before are now clearly evident. The detailed description of the chromosome cycle of C. vermeta gives a clear concept of the meiotic process in euthyneuran gastropods and expands greatly on information contained in any previous report. Early mitotic chromosomes appear as fuzzy, diffuse strands which condense to form mid-prophase chromosomes, the coiled nature of which is evident; the centromeres appear as lightly stained or nonstained areas. Further contraction results in metaphase chromosomes that stain more densely, have smooth margins and centromeres indicated only by constrictions. Anaphase chromosomes are similar but somewhat smaller. The first meiotic prophase nucleus is formed after the last pre-meiotic division. The leptotene chromosomes appear as lightly stained, long, single strands with chromomeres along their length. The free ends show the polarization characteristic of the "bouquet stage". Zygotene pairing begins at the polarized ends and appears to be chromomere-by-chromomere along the length of the homologous strands. Pachytene chromosomes are shorter and more densely stained. Homologues begin to "repel" one another, causing a separation that produces open areas along early diplotene chromosomes. As diplonema progresses, the chromosomes become diffuse and poorly stained. Chiasmata terminalize and contraction continues as the chromosomes form the ring, rod, cross or multiple loop-shaped figures characteristic of diakinesis. Metaphase I bivalents are very condensed ring, half-ring or rod-shaped figures in polar view. Homologous centromeres, with their chromatids, separate at Anaphase I, forming dyad chromosomes about 1/2 the size of Metaphase I bivalents. Following cytokinesis, the chromosomes enter the 2nd meiotic division without an observable period of interkinesis. Prometaphase II dyads, before alignment in the equatorial plane, look like later Metaphase II chromosomes, which are greatly contracted, densely stained and "dumb-bell" shaped. During Anaphase II, each dyad separates to form 2 monads, which pass to opposite poles, where Telophase II is initiated and subsequent cytokinesis takes place. Young spermatids are then formed, each having a conspicuous nuclear membrane. During spermiogenesis, the chromatin becomes disorganized and nuclear condensation takes place to from the mature spermatozoa.