Early Development in the Lancelet (= Amphioxus) Branchiostoma floridae from Sperm Entry through Pronuclear Fusion: Presence of Vegetal Pole Plasm and Lack of Conspicuous Ooplasmic Segregation

Abstract

Lancelet eggs are described from serial fine sections before fertilization and at frequent intervals thereafter until the male and female pronuclei meet at 16 min after insemination. In the unfertilized egg, although mitochondria, as well as yolk granules, are evenly distributed (both are absent only from the egg cortex and meiotic spindle), the mitochondria in the animal third have a more electron-lucent matrix than those elsewhere. The cortex of the unfertilized egg is occupied chiefly by cortical granules, and the subcortical cytoplasm in the vegetal third includes sheets of dense granules interleaved with cisternae of endoplasmic reticulum. By 45 s after insemination, (1) the fertilizing sperm enters (in the animal hemisphere in three out of three observations), (2) yolk granules become patchily distributed around the newly entered sperm, (3) cortical granule exocytosis occurs, and (4) the sheets of dense granules and associated endoplasmic reticulum aggregate with numerous mitochondria into whorls in a yolk-free zone near the vegetal pole. These whorls are the vegetal pole plasm, which is segregated into a single blastomere at each cleavage and might play a role in germ line determination. By 2 min after insemination, the zone of cytoplasm near the animal pole with patchily distributed yolk has enlarged, and the male pronucleus has migrated to the vicinity of the vegetal pole and formed an aster, at the center of which a few mitochondria are aggregated. In lancelets, unlike ascidians, there is no obvious widespread ooplasmic segregation or translocation of cytoplasm from animal to vegetal pole accompanying the movement of the sperm. Between 6 and 16 min, (1) the zone of cytoplasm with patchily distributed yolk enlarges to occupy about the animal third of the egg, (2) the female pronucleus forms by fusion of chromosome-containing vesicles and migrates vegetally, leaving a track of yolk-poor cytoplasm, and (3) the male pronucleus, surrounded by increasing numbers of mitochondria, migrates to meet the female pronucleus just above the equator. In contrast to current opinion, lancelets differ from ascidians both in having a vegetal pole plasm and in lacking marked ooplasmic segregation.

Mechanisms of germ cell specification across the metazoans: epigenesis and preformation

“…Interestingly, an electron-dense region of ooplasm has been reported to localise to a single blastomere at early cleavage stages (Holland and Holland, 1992). Further studies will be necessary to establish whether this blastomere gives rise to the germ cells, which would provide another example of germ cell segregation by preformation.…”

“…Bradley et al, 2001). The dynamics of organelle movement during the assembly of these aggregates also shows striking similarity between different animals (Carré et al, 2002;Heasman et al, 1984;Holland and Holland, 1992). Thus primordial germ cells, as a specialised cell type, may well be homologous across all Metazoa, by the criterion that they have retained an ancestral suite of molecular characteristics that define the germ cell lineage.…”

Dorsal-ventral patterning in amphioxus: current understanding, unresolved issues, and future directions

“…Cortical granules occupy the cortex of the unfertilized egg, and the subcortical cytoplasm in the vegetal third of the egg contains sheets of dense granules that are interleaved with the cisternae of the endoplasmic reticulum (Holland and Holland, 1992;Holland and Holland, 1991). These vegetal structures are associated with numerous ribosomes and mRNAs of germ cell marker genes, such as Vasa, Nanos, Piwil1, Tudor, Pl10, and bruno2 (Dailey et al, 2016;Wu et al, 2011;Yue et al, 2015;Zhang et al, 2013).…”

“…The existing literature presents two opinions that diverge regarding the importance of the entrance of sperm for DV polarity in the amphioxus. One opinion is based on EM observations and proposes that sperm preferentially enter at the animal hemisphere (Holland and Holland, 1992;Holland and Onai, 2012) (Fig. 3A) and after fertilization migrate to the vegetal pole.…”

“…A female pronucleus does not form during this process; instead, there are several vesicles containing the maternal chromosomes that migrate to the cortical cytoplasm about 25° from the animal pole. Here, they join with the sperm nucleus that migrates from the vegetal pole accompanied by a cloud of mitochondria (Holland and Holland, 1992;Holland and Onai, 2012) (Fig. 3A).…”