Non-Canonical Male Meiosis in a Marine Gastropod, Littorina saxatilis

TL;DR

This study reveals unusual stages and patterns in the male meiosis of the marine snail Littorina saxatilis, offering new insights into its reproductive biology.

Contribution

The paper identifies non-canonical meiotic stages and chromomeric patterns in Littorina saxatilis, previously unobserved in this species.

Findings

Non-canonical diffuse and karyosome stages were observed during prophase I of male meiosis in Littorina saxatilis.

Chiasmata appear later than typical, with varied bivalent structures observed during diakinesis and early anaphase I.

Individual bivalents show chromomeric patterns similar to G-banded chromosomes in embryos and spermatogonial cells.

Abstract

The rough periwinkle Littorina saxatilis has become a model species for evolutionary biology in recent decades. The reproductive strategy of this species, including meiosis, is of particular importance. A cytogenetic study using nuclear spreads and live cell observations revealed for the first time some non-canonical features of male meiosis in this marine snail. The most intriguing are the diffuse stage and the karyosome stage during prophase I, as well as the delay in the appearance of chiasmata and chromosome-specific chromomeric patterns of bivalents during diakinesis—early anaphase I.

An atypical course of male meiosis in Littorina saxatilis from zygotene to early anaphase I has been established, which includes non-canonical stages—diffuse and karyosomal. In diakinesis, a structural stepwise transition of bivalents from a single-thread, homogeneously colored form to a double-thread and banded form was discovered. In early diakinesis, in addition to bivalents without pronounced chiasmata, which constitute the majority, rare cruciform short bivalents with one chiasma are revealed. In mid-diakinesis, two or three types of bivalents with one or two chiasmata and several achiasmatic bivalents are identified. In late diakinesis—metaphase I—seven types of bivalents were distinguished, bearing from one to three chiasmata. Some bivalents of the set showed noticeable chiasmata only in early anaphase I. Therefore, the course of diakinesis in L. saxatilis male meiosis is rather atypical. In pachytene, mid- and late diakinesis, and partly in metaphase I, individual bivalents of the L. saxatilis set are reliably identified because they exhibit chromomeric patterns similar to those of the G-banded prometaphase chromosomes of early embryos and spermatogonial mitotic cells. Our research provides a cytological basis for further studies of conservation/variability and evolution of male meiosis.

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