Reproductive biology in the Twentieth Century produced comprehensive descriptions of the mechanisms of egg formation in most of the major orders of insects. While many general principles of ovarian development and physiology emerged, every order turned out to have a set of its own special motifs. Discovery of the lepidopteran motifs is summarized in this essay. The emphasis is on developmental mechanisms, beginning with the early growth and differentiation of female germ cells and ending, after many turns in morphogenesis, physiology and biosynthesis, with eggs that are filled with yolk and encased in chorions. Examples of uniquely lepidopteran traits include the cellular composition of ovarian follicles, the number of tubular ovarioles in which they mature, the functions of cell-to-cell junctional complexes in their maturation, their use of glycosaminoglycans to maintain intercellular patency during vitellogenesis, the role of proton and calcium pumps in their ion physiology, a separate postvitellogenic period of water and inorganic ion uptake, and the fine structure and protein composition of their chorions. Discovery of this combination of idiosyncracies was based on advances in the general concepts and techniques of cell and molecular biology and on insights borrowed from studies on other insects. The lepidopteran ovary in turn has contributed much to the understanding of egg formation in insects generally.

https://bioone.org/journals/Journal-of-Insect-Science/volume-9/issue-50/031.009.5001/Egg-Formation-in-Lepidoptera/10.1673/031.009.5001.pdf

Egg Formation in Lepidoptera

Apes, members of the superfamily Hominoidea, possess a distinctive suite of anatomical and behavioral characters which appear to have evolved relatively late and relatively independently. The timing of paleontological events, extant cercopithecine and hominoid ecomorphology and other evidence suggests that many distinctive ape features evolved to facilitate harvesting ripe fruits among compliant terminal branches in tree edges. Precarious, unpredictably oriented, compliant supports in the canopy periphery require apes to maneuver using suspensory and non-sterotypical postures (i.e. postures with eccentric limb orientations or extreme joint excursions). Diet differences among extant species, extant species numbers and evidence of cercopithecoid diversification and expansion, in concert with a reciprocal decrease in hominoid species, suggest intense competition between monkeys and apes over the last 20 Ma. It may be that larger body masses allow great apes to succeed in contest competitions for highly desired food items, while the ability of monkeys to digest antifeedant-rich unripe fruits allows them to win scramble competitions. Evolutionary trends in morphology and inferred ecology suggest that as monkeys evolved to harvest fruit ever earlier in the fruiting cycle they broadened their niche to encompass first more fibrous, tannin- and toxin-rich unripe fruits and later, for some lineages, mature leaves. Early depletion of unripe fruit in the central core of the tree canopy by monkeys leaves a hollow sphere of ripening fruits, displacing antifeedant-intolerant, later-arriving apes to small-diameter, compliant terminal branches. Hylobatids, orangutans, Pan species, gorillas and the New World atelines may have each evolved suspensory behavior independently in response to local competition from an expanding population of monkeys. Genetic evidence of rapid evolution among chimpanzees suggests that adaptations to suspensory behavior, vertical climbing, knuckle-walking, consumption of terrestrial piths and intercommunity violence had not yet evolved or were still being refined when panins (chimpanzees and bonobos) and hominins diverged.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/joa.12454

Why are there apes? Evidence for the co-evolution of ape and monkey ecomorphology

Summary Caste-specific differentiation of the female honey bee gonad takes place in the fifth larval instar. In queen larvae most ovarioles exhibit almost simultaneous formation of numerous germ cell clusters within the first 20 h after the last larval molt. Ultrastructurally distinctive fusomal cytoplasm connects these cystocytes. Germ cell differentiation is accompanied by morphological changes in somatic components of the ovarioles, the follicle and the terminal filament cells. Subsequently, queen ovarioles elongate and differentiate basal stalks that coalesce in a basal calyx. A second round of mitotic activity was found to occur in the late prepupal and early pupal queen ovary. This round may elevate germ cell numbers composing each cluster to levels observed in follicles of adult honey bee queens. In contrast, germ cell cluster formation does not occur in most of the 120–160 ovarioles of the larval worker ovary, but instead many cells in such ovarioles show signs of impending degeneration, such as l...

Germ cell cluster formation and cell death are alternatives in caste-specific differentiation of the larval honey bee ovary

Higher insects have evolved a highly effective mechanism for loading their unfertilized eggs with long-lived messenger RNAs, ribosomes, transfer RNAs, enzymes, and less complex organic molecules. These are utilized later for synthesizing the proteins needed by the young embryo. The mechanism generates endopolyploid nurse cells that produce the required compounds and a system of canals through which these products are exported to the oocyte.

The Formation of Clones of Interconnected Cells during Gametogenesis in Insects

Publisher Summary This chapter focuses on spindle architecture in Lepidoptera. The variability in spindle structure is illustrated by describing meiosis as well as mitosis in both sexes. Lepidoptera chromosomes are unusually small, and light microscopy has not contributed much to knowledge of chromosome structure in this group. Although observations are limited to a few species, fine structure studies suggest that Lepidoptera chromosomes represent a type intermediate between monokinetic and holokinetic chromosomes. Four different spindle types have been identified in Lepidoptera. These are somatic and gonial mitoses, female meiosis I, and divisions in eupyrene and apyrene spermatocytes. Though smaller, the spindles of spermatogonia and somatic tissues of Lepidoptera resemble those of mammalian cells. Meiosis I in female Lepidoptera is achiasmatic. Eupyrene spindles of Lepidoptera, which give rise to fertile spermatozoa, usually show a prominent spindle envelope and a rich intraspindle membrane system. Apyrene spindles, which produce sterile sperm, possess fewer membranes and microtubules (MTs) than the spindles of the “sheathed nuclear division.” Segregation of the chromatin is highly irregular in the apyrene line. The idea is advanced that the evolution of the sheathed nuclear division contributed to the development of double spermatogenesis in Lepidoptera. The acetylation status of MTs in mitotic and eupyrene meiotic spindles and its implications for MT behavior are described in the chapter.

The Unique Structure of Lepidopteran Spindles

Intercellular bridge development was compared in Ceropia by transmission electron microscopy in the germ cells of males and females. Bridge formation begins in the fourth instar, and the spindle remnants in newly formed bridges are replaced at this time by an amorphous material known as the fusome. In the four- and eight-celled cystocyte clusters of the female, the newly formed intercellular bridges migrate centripetally, forming a central complex of bridges surrounded by a rosette of germ cells. The fusomes become continuous and occupy all of the bridges in the complex. At each division each mitotic spindle orients itself with one pole toward the continuous fusome. In the female, mitosis stops at the eight-cell stage, and the cystocytes all enter first meiotic prophase. At the end of the fifth instar, when nurse cell differentiation commences in seven of the cells, the continuous fusome is replaced by a continuous mass of mitochondria and microtubules. In the male, bridge migration and rosette formation are abandoned during the later mitotic and meiotic divisions; during these stages the fusome is no longer continuous. The fusome of males disappears during spermatid differentiation and is not replaced by mitochondria and microtubules. The ability of the centrally located continuous fusome to orient the mototic spindles of succeeding mitoses could account for the earlier observations that all pre-existing bridges remain in only one of the daughter cells at each successive cystocyte division in the female.

Isabel Mandelbaum

Papers

Intercellular bridges and the fusome in the germ cells of the Cecropia moth