Germ plasm provides clues on meiosis: the concerted action of germ plasm granules and mitochondria in gametogenesis of the clamRuditapes philippinarum

Abstract

SummaryGerm plasm-related structures (GPRS) are known to accompany meiotic cell differentiation but their dynamics are still poorly understood. In this study, we analyzed the ultrastructural mechanisms of GPRS transformation during oogenesis and spermatogenesis of the bivalve mollusc Ruditapes philippinarum (Manila clam), exploring patterns of GPRS activity occurring at meiosis onset, sex-specific difference/similarity of such patterns, and the involvement of mitochondria during GPRS-assigned events. In the two sexes, the zygotene–pachytene stage of meiosis is anticipated by three shared steps. First, the dispersion of germ plasm granules containing the germ line determinant VASA occurs. Second, the VASA protein deriving from germ plasm granules enters neighbouring mitochondria and appears to induce mitochondrial matter release, as supported by cytochrome B localization outside the mitochondria. Third, intranuclear VASA entrance occurs and the protein appears involved in chromatin reorganization, as supported by VASA localization in synaptonemal complexes. In spermatogenesis, these three steps are sufficient for the normal course of meiosis. In oogenesis, these are followed by the action of ‘germ plasm granule formation complex’, a novel type of structure that appears alternative to the Balbiani body. The possibility of germ plasm involvement in reproductive technologies is also suggested.

Morphogenesis of the Balbiani body in developing oocytes of an orthopteran, Metrioptera brachyptera, and multiplication of female germline mitochondria

“…It is to the point to add here that intimate association of nuage and mitochondria has been reported in developing oocytes of various vertebrate and invertebrate species (see Voronina et al, 2011; Kloc et al, 2014 for further details). Moreover, in some species spatial arrangement of mitochondria and nuage is almost identical to that described in M. brachyptera (e.g., Brinton & Oliver, 1971 and figure 9 therein; figures 5 and 6 in Dohmen, 1985; figure 4 in Smiley, 1990; figure 5 in Falleni, 1993; figures 2 and 4 in Gonobobleva & Efremova, 2017; figure 4b in Reunov et al, 2019). On the other hand, parallel nuage filaments encompassing and combining neighboring mitochondria have never been noticed, even in detailed TEM studies.…”

“…The structure of the Bb may differ even in related animal taxa, and is rather dynamic in subsequent phases of Bb morphogenesis. In some species (frogs, chelicerates, myriapods), the Bbs are large, solid, and persist in the ooplasm till advanced vitellogenesis; in others (insects, clams), they are less conspicuous, transitory, and sometimes even dispersed (Kloc et al, 2014; Reunov, Alexandrova, Reunova, Komkova, & Milani, 2019). In spite of relatively numerous studies, the functioning of the Bb still remains enigmatic.…”

“…Accumulations of the nuage are characteristic and ubiquitous constituents of all animal germline cells. The molecular composition of the nuage is still enigmatic, it has been shown, however, that in the model organism and certain other invertebrates it comprises proteins (e.g., Vasa) involved in the specification of the germline cells (see Jaglarz, Kloc, Jankowska, Szymanska, & Bilinski, 2011; Voronina, Seydoux, Sassone‐Corsi, & Nagamori, 2011; Kloc et al, 2014; Reunov et al, 2019 for further details). In addition to mitochondria and nuage, the Bb may also comprise various organelles such as centrioles, elements (cisternae, vesicles) of rough or smooth endoplasmic reticulum (er), Golgi complexes (Gc), lipid droplets, and cytoskeletal fibers (Jedrzejowska & Kubrakiewicz, 2007, 2010; Kloc et al, 2004, 2014).…”

Germ plasm-related structures in marine medaka gametogenesis; novel sites of Vasa localization and the unique mechanism of germ plasm granule arising

“…However, there are examples showing that CB is absent during spermatogenesis although the GG, IMC, and mitochondrial clusters are normally present during early meiosis. CB were not detected in spermatogenesis of sipunculids (Reunov and Rice, 1993), nemerteans (Reunov and Klepal, 1997), polychaetes (Reunov et al, 2010), bivalve molluscs (Reunov et al, 2018), phoronids (Reunov and Klepal, 2004), brachiopods (Hodgson and Reunov, 1994), and echinoderms (Au et al, 1998). Therefore, in the animal kingdom there are two patterns of spermatogenesis that are uniform…”

Cell cultures of the Manila clam and their possible use in biomonitoring and species preservation

“…The described protocol can be used to obtain cell cultures useful for investigations in different fields of science: e.g. longevity determinants (Rabinowitz 2010) or germline cell differentiation and reproductive biology (Reunov et al 2019).…”