Budding and the Reproductive Cycle of Distaplia

Abstract

The complete developmental cycle, sexual and asexual, is described for several species of the ascidian genus, Distaplia. Part I. The cycle of D. rosea is described, from the active larva to the establishment of the first blastozooid generation. Of the four or five buds produced from the larval probud, only one develops, the rest remain abortive. Some larvae are often retained in the colony after the dissolution of the parent zooid and give rise to blastozooids (Text-figs. 1-3). The general morphology of mature zooids is described for D. bermudensis, and their arrangement into typical systems in the colony. Zooids are at first functional males, while the brood-pouch develops later as the first egg in the ovary approaches maturity. The brood-pouch is essentially a loop of the oviduct and does not involve the atrial wall (Text-figs. 4-5). In the gross development of the bud up to the time it becomes functional, the originally spherical bud elongates relatively along its antero-posterior axis but otherwise does not exhibit differential growth. After the functional state is reached, differential growth of the atrial siphon commences and eventually transforms the small circular siphon into the relatively enormous atrial aperture of the mature zooid (Text-figs. 4 and 6). Gonads develop as hermaphrodite structures in buds of D. rosea, D. bermudensis, D. occidentalis, and D. clavata, but as either ovaries or testes alone in D. magnilarva (Text-fig. 7). Part 2. Tadpole larvae of Distaplia vary greatly in size according to the species, but not in organization, the cell number of purely larval structures such as adhesive organs and sensory organs being constant (Text-fig. 8). Cleavage of the egg, in spite of large size and yolk content, is typical of ascidians in general, and shows bilateral patterning comparable with that of Styela and Ciona (Text-fig. 9). There is great growth in the development from the egg to the tadpole stage, though the full number of cells of both the notochord and tail muscle is reached at an early stage, about 40 in the case of the notochord (as in all other ascidians) and over 2,000 tail-muscle cells (the largest number known). In the later development of the tadpole larva, the anterior region bearing the adhesive organ and the ampullae becomes progressively constricted off until it remains attached only by a vascular stolon. The probud is separated from the abdomen at a precise stage in the whole development and subdivides before tadpole development is complete (Text-figs. 10-12). The sensory vesicle contains a typical unicellular otolith and an ocellus consisting of optic pigment cup, layer of retinal cells, and three unicellular lens cells. In the tail the intracellular notochordal vacuoles run together to form a continuous fluid cylinder with the chordal cells forming a limiting sheath. Myofibrillae are continuous from cell to cell in the lateral interfaces of the ectoplasmic regions (Text-fig. 13). Part 3. Contrary to all previous reports, buds are produced by adult zooids and are not confined to the tadpole larva, but they are produced at a different phase of the life-cycle of the individual zooid. After sexual reproduction is complete, the zooids atrophy, their tissues autolysing and becoming phagocytosed in part. All of the zooid degenerates with the exception of the epicardium and surrounding epidermis. This structure actually undergoes extensive elongation, at the close of which the posterior end of the pericardium and enclosing epidermis constricts off as one of two probuds (Text-figs. 14-15). This is essentially the same process, apart from the stage with which it is associated, as occurs in the tadpole larva. In both cases the probud elongates and constricts into two or more definitive buds (Text-figs. 16-17), the smallest of which fail to develop. The formation of hermaphrodite or unisexual gonads depends primarily on the mass of the presumptive gonad tissue segregated at a very early critical period in bud morphogenesis (Text-fig. 18). The process of bud formation is compared with that of the allied genus Colella, and an interpretation very different from that of Brien is presented (Text-fig. 20).