Structure of the apical cells of the testis of the tenebrionid beetles: Tenebrio molitor and Zophobas rugipes

Abstract

The developmental cytology of the apical tissue of the testis of Tenebrio nzolitor and Zophobas rugipes was studied with light and electron microscopy. In the early larvae of both species the tisue was found to be a thickened protrusion of nongerminal cells appearing at the apical end of each testis follicle following gonadal differentiation. The cells persist through pupal and adult stages in both species, becoming more prominent at these stages in Z. rugipes, despite tracheal invasion in both species. In older adults the apical tissue regresses and ultimately distintegrates.Ultrastructurally the apical cells are distinguished from adjacent germinal cells by their (1) small, rounded or oval nuclei, (2) highly convoluted plasma membrane, ( 3 ) electron-opaque cytoplasm, (4) profuse concentrically-stacked, granular endoplasmic reticulum, ( 5 ) large aggregates of glycogen-like granules, (6) numerous small, tubular mitochondria, ( 7 ) well-developed Golgi centers and ( 8 ) striking arrays of microtubules. These cells have many cytological features in common with the androgenic gland cells of crustaceans, but not with the steroidogenic cells of vertebrates. Evidence for the formation of protein granules is also lacking. As yet, experimental procedures have not indicated an endocrine function for these cells in tenebrionids. However, their cytology is consistent with secretory activity of some kind.The apical tissue of the insect testis consists of a group of cells, or a single cell with abundant cytoplasm, situated at the apex of each testicular follicle. During the early part of this century (see Carson, '45; Aboim, '45, for reviews) much attention was paid to the cytology of the apical cells, and several hypotheses were presented as to their origin, function and fate. In most insects examined the apical cells were considered to be larval structures. Recently, Naisse ('65) has analogized the apical cells of a coleopteran Lampyris noctiluca with the androgenic gland of crustaceans. In Lampyris implantation of the larval testis at a critical stage into a female larva will completely transform the sex of the genetically female host at primary, secondary and behavioral levels. Naisse attributes the sex reversal to a hormone secreted by the apical cells, under the control of neurosecretory cells of the pars intercerebralis of the male larval brain (Naisse, '65).This important experimental finding and the absence of any ultrastructural information on the apical cells prompted this study of the fine structure of these cells in two tenebrionid beetles : Tenebrio molitor and Zophobas rugipes. The apical tissue of Tenebrio molitor had been described as early as 1902 by Demokidoff; however, he called the tissue a "lens" and claimed that it was different from the apical cells (of Verson) seen in other insects. Edwards ('61) comments similarly on the "lens" in another coleopteran, the cerambycid Prion,oplus.In the present study attention is directed a t changes in the apical cells during larval development, and to a com...

Differentiation of male reproductive system inOryctes rhinoceros (Coleoptera: Scarabaeidae)

“…However, these structures become distinct only in the late third instar larva, as reported for T. molitor (Menon 1969) and P. anxia (Berberet and Helmes 1972). However, in some Coleoptera these structures became apparent either at prepupal or pupal stages (Metcalfe 1932;Elliott 1964).…”

“…According to Gundevia and Ramamurthy (1974) Verson's cells encircle apical cell and are nutritive in function. The ultrastructural studies on apical cell (Menon 1969;Richard-Mercier 1981) also show that they are nutritive in function. Apical cell is not observed in Tribolium castaneum (Murad and Ahmad 1977) and in P. punctatipennis (Inamdar and Joshi 1986).…”

“…Spermatogenesis in O. rhinoceros is initiated at the late larval instar as observed in T. malitor (Menon 1969). According to Virkki (1967) spermatogenesis in many melolonthines commenced in the pupal stage and is completed before adult emergence.…”

“…Virkki (1957Virkki ( , 1967 studied the structure of testis follicles and meiosis in scarabaeid beetles while Landa (1959) explained the spermatogenesis in adults of Melolontha melolontha. Menon (1969) studied the differentiation of testis and apical cells in T. molitor. Spermatogenesis in Phyllophaga anxia (Berberet and Helmes 1972) has been reported.…”

LE TISSU APICAL DES GONADES DU DORYPHORE,LEPTINOTARSA DECEMLINEATASAY (COLEOPTERA: CHRYSOMELIDAE)

“…Ce tissu est lui-meme controle par une neurohormone issue des cellules neurosecretrices protocerebrales 'a petits grains'. Chez deux autres Coleopteres, Menon [9] signale que les greffes de testicules larvaires ne modifient pas la differenciation sexuelle des larves hates femelles. Nos travaux, realises chez le Doryphore, Leptinotarsa decemlineata n'ont pas confirme le role androgene du tissu apical: les greffes de testicules larvaires dans les larves n'ont pas induit la masculinisation des ovaires [14].…”