Further information concerning the envelopes surrounding dipteran eggs

Transcription

1 209 Further information concerning the envelopes surrounding dipteran eggs By R. C. KING (From Northwestern University, Evanston, Illinois, U.S.A.) With z plates (figs, i and 2) Summary A centripetal migration of follicle cells, which results in a separation of egg and nurse chambers, occurs in higher dipterans like Drosophila melanogaster but not in lower dipterans like Anopheles maculipennis. In Anopheles and Drosophila, while the vitelline membrane can be secreted in the absence of the oocyte, the follicle cells must be present. This suggests that the follicle cell is the governing agent in the synthesis of the vitelline membrane. The envelopes of Drosophila embryos differ from those surrounding ovarian eggs in that a membrane about 0-04 fi thick lies directly outside the vitelline membrane. This thin layer is thought to represent a waterproofing wax which forms once the egg leaves the ovariole. Introduction THE mature oocyte of most dipterous insects is surrounded by an inner vitelline membrane and an outer chorion. King and Koch (1963) demonstrated for Drosophila melanogaster that the vitelline membrane is an intercellular structure which is always bordered on the outer surface by the plasma membranes of follicle cells. The inner surface is usually bordered by the oocyte plasmalemma; but under abnormal circumstances a plasma membrane supplied by another type of cell can serve the same function. The fact that vitelline membrane can be secreted in the absence of an oocyte but only when follicle cells are present suggested that the follicle cell plays the major role in the secretion of precursor material of the vitelline membrane. Data dealing with mosquito oogenesis will be presented in this paper which support this contention. King and Koch also presented information which suggested that a waterproofing wax is laid down between the vitelline membrane and the chorion once the egg leaves the ovariole. Further data bearing on this waterproofing layer are presented here. Material and methods The slides of sectioned ovaries from Anopheles maculipennis examined in this study were the same that Dr. A. J. Nicholson used for his classical paper (1921) on the oogenesis of this mosquito. Newly laid embryos of wild type Drosophila melanogaster were fixed for 15 min in 1% buffered OsO 4 at 50 0 C. The dehydration and infiltration procedures of Staubli (1963) were followed to produce plastic embedments. Thin sections cut from these blocks with an ultrotome were photographed with a Siemens elmiskop I. [Quart. J. micr. Sci., Vol. 105, pt. 2, pp , 1964.]

2 21 o King Envelopes of dipteran eggs Results and conclusions Differences between Anopheles and Drosopkila in the behaviour of ovarian follicle cells In the Lepidoptera, the Hymenoptera, the adephagous Coleoptera, and in the higher Diptera, the nurse cells are gradually separated from the oocyte by a wedge-shaped layer of follicle cells which advances centripetally from the lateral follicular envelope (see fig. 2 of King and Koch, 1963). The vitelline membrane is extended inwards by these cells, and eventually the oocyte is completely surrounded by this envelope. The centripetal migration of follicle cells does not occur in mosquitoes (see figs. 1 to 5 of Nicholson, 1921). Instead, as the growth rate of the oocyte exceeds that of the nurse cells, the ooplasm extends laterally between the follicular envelope and the cluster of 7 nurse cells. Eventually this group of nurse cells is completely enveloped by ooplasm save for a narrow isthmus which connects it to the exterior (fig. 1, D). At a later stage (fig. 1, E) the nurse cell nuclei are found outside the oocyte, and presumably these have been forced out through the isthmus by the pressure exerted by the growing ooplasm. The vitelline membrane (or the 'inner wall' as Nicholson called it) forms by the fusion of small droplets which accumulate between the follicular epithelium and the oocyte. This membrane is clearly visible in all the photomicrographs of fig. 1. A region of particular interest is shown in fig. 1, A and at higher magnifications in fig. 1, B and c. It is clear that in this region precursor droplets for the future vitelline membrane have formed at an interface between follicle cells and nurse cells. Thus as in Drosophila there is evidence pointing to the follicle cell as the controlling agent in the secretion of the vitelline membrane. It should be recalled that Nicholson came to the same conclusion and for an equally good reason. He noticed at a later stage in oogenesis that an isolated deposit of material with staining properties similar to vitelline membrane was formed between certain specialized follicle cells and the nurse cells. This deposit he called the 'stopper', since it was destined to form a plug below the micropylar perforation of the vitelline membrane (see figs. 39 and 41 of Nicholson, 1921). The stopper also appears in fig. 1, D and E. From a comparison of the morphology and development of the egg shells in the 2 species, it is clear that the endochorion of Drosophila corresponds to the inner portion of the outer wall of the Anopheles egg. There is no structure in the Drosophila egg shell that can be homologized with the floats FIG. I (plate). Light photomicrographs of sections of egg chambers of Anopheles maculipennis. A, B, D, and E, phase contrast; c, bright field. A, D, and E, show a series of advancing stages in the development of the mosquito egg chamber. B and c are higher magnifications of A. Note how the cluster of nurse cells become enveloped by ooplasm (A VS D). At a later stage (E) the degenerating nurse cell nuclei lie outside the oocyte. In figs, B and c arrows point to precursor droplets for the future vitelline membrane which have formed between the follicle cells and the nurse cells. The nurse cell nuclei contain banded polytene chromosomes and a large nucleolus. /, follicular epithelium; o, ooplasm containing yolk spheres; n, nurse cell cluster; s, stopper; v, vitelline membrane.

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5 King Envelopes of dipteran eggs 211 of Anopheles. However, the cells forming the floats resemble the cells which produce the chorionic appendages in Drosophila in that both omit vitelline membrane production and proceed directly with the secretion of endochorion. The envelopes of Drosophila embryos The shell surrounding the young embryo is similar to that of the ovarian egg (contrast fig. 2 of the present paper with fig. 8 of King and Koch, 1963). There is an exochorion of variable thickness (generally between 0-5 and 1 /x). The outer endochorion is 0-2 JU. thick, and the air-filled endochorionic space averages about 1 (i in width, although it varies considerably. An inner endochorion about 0-04 fm thick lies above a slightly thicker membrane which is thought to be the remains of the fused plasma membranes of the dead follicle cells. Below this lies a structure seen in the embryo, but not in the ovarian egg; a membrane which has a thickness of about 0-04 /n. This layer presumably represents the waterproofing wax. The vitelline membrane is 0-3 to 0-4 {i thick. In places where the vitelline membrane is pulled apart from the endochorion the layer of wax follows the convolutions of the vitelline membrane and is generally separated from it by a space about 0-05 JU, wide. The embryo also differs from the ovarian egg in that the cellular covering of the exochorion is missing from most of its surface. The waxy layer appears in a recently published electron micrograph of Mahowald (1962, his fig. 1). This research was performed during my tenure of a National Science Foundation Senior Postdoctoral Fellowship at the Division of Entomology, Commonwealth Scientific and Industrial Research Organization, Canberra, Australia while on leave from Northwestern University. I am grateful to the Division Chief, Dr. Douglas Waterhouse, for his generous hospitality, to Professor Edgar Mercer for making available the electron microscope facilities of the John Curtin School of Medical Research, and to Dr. A. J. Nicholson for letting me examine his Anopheles slides and for his useful comments upon the finished manuscript. Excellent technical assistance was performed by Mr. Surinder Aggarwal. References King, R. C. and Koch, E. A., Quart. J. micr. Sci., 104, 297. Mahowald, A. P., J. exp. Zool., 151, 201. Nicholson, A. J., Quart. J. micr. Sci., 65, 395. Staubli, W., J. Cell Biol., 16, 197. FIG. 2 (plate). An electron micrograph of a segment of the envelopes surrounding an early embryo of Drosophila melanogaster. end^, outer endochorion; end a, inner endochorion; endp, endochorionic pillar; ends, endochorionic space; ex, exochorion; o, ooplasm; v, vitelline membrane; wl, waxy layer.