The endocrine control of reproduction in female insects has been relatively well studied, perhaps because egg production is often cyclical and the experimental manipulation of hormones at key developmental periods has a direct effect on the number of offspring the females produce. This has not been the case for the males of most insect species in which the production of sperm may begin early during the larval, nymphal, and pupal stages and often continues throughout adult life. Given the vastly different hormonal conditions that exist during the immature and adult periods, a unifying scheme for the control of male spermatogenesis has not been forthcoming. There are only a few examples of hormones controlling the reproductive processes of male insects.
PRODUCTION OF SPERM
The male gametes are produced in the follicles of the paired testes. The development of the testes during metamorphosis and their synthesis of proteins respond to the insect hormone 20-hydroxyecdysone (20HE). In the reproductively mature male, apical stem cells at the anterior end of the testes are surrounded by cyst progenitor cells and specialized non-dividing somatic cells, known the hub, that together form the stem cell niche. Signaling from the niche regulates the renewal, maintenance, and survival of the stem cells. The stem cells divide mitotically to produce a gonialblast that becomes surrounded by a follicular cyst produced by the cyst progenitor cells. The development and maturation of spermatozoa occurs within these cysts.
Both 20HE and juvenile hormone (JH) have been implicated in the regulation of spermatozoa within the testes. In the absence of the pro-thoracic glands of the adult male, the 20HE is produced by the testes themselves in response to an ecdysiotropin produced by the neurose-cretory cells of the brain. High levels of 20HE increase the rate at which the spermatogonia undergo mitotic divisions to form spermatocytes, but this increase is abolished by high levels of JH. When spermatogenesis begins during the larval period, as in some Lepidoptera, the spermato-cytes initiate their meiotic divisions but are arrested at prophase until the end of the larval period is reached. In many holometabolous insects, a peak of 20HE occurs before the end of the larval period that induces the “wandering” behavior that allows the larva to find a secluded spot in which to pupate. A postwandering peak of 20HE unblocks the mei-otic division of the spermatocytes and allows the cells to proceed to metaphase. In some insects, JH has also been found to accelerate sper-matogenesis. The release of mature spermatozoa from the cysts has a circadian rhythmicity in some insects that is inhibited by 20HE and occurs when levels of 20HE decline.
When spermatogenesis begins during the larval stage, as it does in most Lepidoptera, it is interrupted if the insect diapauses as a larva or pupa. However, it resumes once diapause has been completed. The interruption is not caused by a pause in developmental activity but rather from the lysis of developing gametes before they become mature. The renewal of spermatogenesis when diapause has been terminated occurs when the titers of 20HE increase.
The developmental sequence in which spermatozoa develop from spermatids involves an elongation of both the nucleus and the flagellum. In Lepidoptera, the nuclear elongation is triggered by the declining concentrations of 20HE, but the elongation of the flagel-lum appears to be independent of any hormone.
Lepidopteran males produce two different types of sperm. Eupyrene sperm have a nucleus and fertilize the eggs, but apyrene sperm are anu-cleate. Both types of sperm are transferred to the female spermatheca after copulation. The function of the apyrene sperm is not well understood but they may provide nutrients required by the eupyrene sperm or aid in the migration of the eupyrene sperm within the female genital tract. The developmental pathway leading to the differentiation of apyrene sperm is regulated by a hemolymph-borne apyrene-spermatogenesis-inducing factor that is present at the time of pupation.
ACCESSORY GLANDS
The accessory glands of the male reproductive system produce semen, accessory structures such as spermatophores, and various pep-tides that regulate female behavior and physiology. The interaction between JH and 20-hydroxyecdysone during postembryonic development regulates the development and differentiation of the glands. Juvenile hormone alone may also control the synthesis of those specific proteins that are transferred to the female. The accumulation of some secretory peptides in the glands that are enhanced by JH may be either enhanced or inhibited by the simultaneous presence of 20HE. In the blood sucking bug, Rhodnius prolixus, protein synthesis by the glands is stimulated by brain neuropeptides. In the German cockroach, Blattella germanica, the activity of the corpora allata, which is the source of JH, declines during the formation and transfer of the spermatophore and may thus initiate a new cycle of male accessory gland maturation.
