The Biological Bulletin, Vol 182, Issue 1 77-96, Copyright © 1992 by Marine Biological Laboratory

DEVELOPMENT AND REPRODUCTION

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

L. Z. Holland and N. D. Holland
Marine Biology Research Division, Scripps Institution of Oceanography, University of California San Diego, La Jolla, California 92093-0202

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.

This article has been cited by other articles:

				L. Z. Holland and N. D. Holland A revised fate map for amphioxus and the evolution of axial patterning in chordates Integr. Comp. Biol., September 1, 2007; 47(3): 360 - 372. [Abstract] [Full Text] [PDF]

				C. G. Extavour and M. Akam Mechanisms of germ cell specification across the metazoans: epigenesis and preformation Development, December 15, 2003; 130(24): 5869 - 5884. [Abstract] [Full Text] [PDF]