A Cinematographic study of embryonic developmentin vitro of the viviparous, paedogenetic gall midgeHeteropeza pygmaea

Abstract

1. In the paedogenetic gall midgeHeteropeza pygmaea, embryonic growth is at the expense of maternal tissues. The possibility of culturing egg folliclesin vitro throughout the entire period of embryonic development allowed the filming of embryogenesis. In the present paper development, growth and degeneration of egg folliclesin vitro (at 25° C) are analysed by time-lapse film. 2. During development of the mature egg follicle up to germ band formation, the yolk globules undergo alternative periods of oscillation and rest within the yolk syncytium. During the periods of time in which the yolk globules are at rest, cleavage divisions take place. All egg follicles analysed showed 13 resting periods which corresponded to the 13 cleavage divisions. A comparison with other investigations indicates, that oscillation of the yolk globules is not required for the migration of the nuclei, but is connected with a special function of the yolk syncytium in paedogenetic gall midges. 3. From the 1st until the 6th cleavage division the average duration of the mitotic cycle decreases from 75 to 50 minutes; from the 6th until the 13th cleavage division it increases to more than 3 hours. 4. Blastokinesis, i. e. germ band extension and germ band retraction, in all probability is the consequence of autonomous movements of the germ band and not of a morphogenetic effect of the yolk syncytium. 5. Egg follicles from different preparations show varying rates of development and growth whereas egg follicles within one culture drop develop and grow with the same rate although they may not be at exactly the same stage of development. 6. In certain stages of development (cleavage, germ band retraction and dorsal closure) the increase in length of the egg follicle is discontinuous. During germ band retraction many egg follicles show up to 7 elongations and contractions, which may amount to as much as one fifth of the egg follicle length. The period of time of one elongation-contraction cycle is between 1 1/4 and 2 3/4 hours and increases by 1/4 hour with each new cycle. At the same time as the egg follicle length increases, its width decreases and vice-versa, suggesting that the increase in volume is continuous. 7. Measurements of two egg follicles at the blastoderm stage revealed rhythmic fluctuations in length which amounted to no more than one fiftieth of the total egg follicle length. There may be as many as 60 such fluctuation cycles, each of which has a constant period of about 15 minutes. The endogenous process underlying these fluctuations is obscure. 8. The egg follicles which degenerate in culture are generally the smaller and less developed ones in any given preparation; however, until their sudden degeneration, they show the same rate of growth and development as the non degenerating egg follicles. 9. The extraordinary mode of paedogenetic egg development ofHeteropeza is interpreted as a displacement of embryogenesis into oogenesis.

Paedogenesis in the dipteran insectHeteropeza pygmaea:an interpretation

“…In our opinion, the extraordinary nature of paedogenetic egg development can be interpreted rather as a shifting of embryogenesis into oogenesis [7]. We present here evidence supporting this interpretation, obtained by a comparison of oogenesis in larval reproduction with that in imaginal reproduction.…”

“…The method for culturing Heteropeza embryos in vitro has been described previously [7]. Ovaries and follicles, likewise, can be cultured in vitro [10] under female-or maledetermining conditions [11].…”

“…The reserve material in mature eggs, accumulated during the short period of egg formation, contains mainly fatty yolk and no protein yolk [ 9]. A chorion is not produced; the eggs continue to grow, and the follicular epithelium persists till the end of embryonic development [ 7]. The increase in size of H. pygmaea eggs in the course of embryonic development is about 200 times (Fig.…”

“…The size of the female-determined follicles at release from the ovaries is about 40-50 J.J.m. Genital ducts are not developed in paedogenetic reproduction; the follicles spread in the haemolymph from where they take up nutrients [7]. The nurse chamber probably supplies the oocyte with RNA [9], and degenerates only during early embryonic development.…”

“…Floating in the haemolymph of the mother larva the follicles grow, and the oocytes undergo maturation. On completion of meiosis, cleavage sets in; embryonic development still takes place within the body of the feeding and growing mother larva [7]. By the time the embryos have completed embryogenesis, the mother larva (actually a hemipupa Downloaded by [New York University] at 06:43 16 May 2015 enclosed by the larval exuvia [8]) has died, and nearly all her tissues have been histolysed.…”