Cytodiffrentiation in the accessory glands ofTenebrio molitor. VI. A congruent map of cells and their secretions in the layered elastic product of the male bean-shaped gland

Male Accessory Glands of Blister Beetles and Cantharidin Release: A Comparative Ultrastructural Analysis

Simple Summary

Meloidae, also called blister beetles, are known to actively produce cantharidin, a toxic terpene with a defensive function that is released externally by reflex bleeding, and that is also stored in large quantities in the male accessory glands. These glands are involved in the transfer of terpene from males to females, which receive cantharidin via spermatophores as a nuptial gift to be used for their own protection and that of the eggs. However, it is still debated whether the male accessory glands can actively produce the terpene or if they only mediate its transfer, since neither the cantharidin-producing organ nor the metabolic pathway are known to date. The focus of the work is to analyze comparatively the accessory glands of males in representative Meloidae species to provide morphological evidences that can contribute to this debate. The results highlight the complexity of the accessory gland system, consisting of three different types of glands that are highly variable between species with the exception of one, which remains conserved even in independent phyletic lines. This gland is a good candidate for hypothesizing a direct role in cantharidin production and/or concentration.

Abstract

Members of the family Meloidae are known to produce cantharidin, a highly toxic monoterpene found in their hemolymph and exuded as droplets capable of deterring many predators. As a nuptial gift, males transfer large amounts of cantharidin to females via a spermatophore, which is formed by specific accessory glands containing high concentrations of this terpene. Using light, electron and ion beam microscopy, the ultrastructural features of the three pairs of male accessory glands as well as the glandular part of the vasa deferentia were comparatively investigated in seven species of blister beetles belonging to five different tribes and two subfamilies. All gland pairs examined share common features such as mesodermal derivation, the presence of muscle sheath, a developed rough endoplasmic reticulum, abundant mitochondria, secretory vesicles, and microvillated apical membranes. Within the same species, glands exhibit distinctive features, suggesting that each pair is responsible for the formation of a specific substance. The vasa deferentia, while showing many similarities within the family, often exhibit features unique to each of the individual species investigated, whereas the accessory glands of the first and second pairs display the highest degree of ultrastructural variability. A comparison across the species shows an interesting constancy limited to ultrastructural features in the third pair of accessory glands. The similarities and differences among the species are discussed in the light of the available literature and in relation to the potential role that blister beetles’ male accessory glands could play in the storage and management of cantharidin.

The male reproductive accessory glands of the blister beetle Meloe proscarabaeus Linnaeus, 1758 (Coleoptera: Meloidae): Anatomy and ultrastructure of the cantharidin-storing organs

Blister beetles owe their name to their ability to release cantharidin, a blistering terpene, the highest concentration of which is retained in male accessory glands. The anatomy and ultrastructure of the three pairs of male reproductive accessory glands and the glandular region of the two vasa deferentia of Meloe proscarabaeus were investigated using light, electron and ion beam microscopy. All of the mesodermal glands here analysed share a common structural organization with an outer muscular layer and an inner glandular epithelium facing a broad lumen in which the secretory products are released. Developed rough endoplasmic reticulum, Golgi systems, abundant mitochondria, numerous secretory vesicles and a microvillated apical membrane are commonly found in the cells of different glandular epithelia, suggesting that all accessory gland pairs as well as the vasa deferentia are involved in an active synthesis. Nevertheless, each pair of glands appears specialized in the production of a specific set of substances, as suggested by the peculiarities in cellular ultrastructure and by the different aspect of the secretions stored in their glandular lumen. The above cited features of male accessory glands of M. proscarabaeus are compared with those of other beetles and some hints on their potential role in producing and/or concentrating cantharidin are provided.

Ultrastructural localization of basic proteins and carbohydrates in male accessory glands of two Triatoma species (Hemiptera, Reduviidae, Triatominae)

The male accessory glands in Triatoma are tubular and produce substances with some functions related to production of the spermatophore. In the current study, the cytochemistry of male accessory glands was evaluated in starved Triatoma brasiliensis and adult Triatoma melanica. The storage of carbohydrates and proteins in T. melanica male accessory glands occurs earlier than in T. brasiliensis. In addition, the occurrence of eletron-lucent granules without carbohydrates and proteins suggests that other substances are released by these glands, which may be used for lubrication of the male genitalia. Male T. brasiliensis has more intense secretory activity in the fifth day of adult life, which may indicate a higher reproductive capacity. The analysis of lipid production in male accessory glands can contribute to the knowledge of spermatophore formation in these species.

An integrative view of sexual selection in Tribolium flour beetles

Sexual selection is a major force driving the evolution of diverse reproductive traits. This evolutionary process is based on individual reproductive advantages that arise either through intrasexual competition or through intersexual choice and conflict. While classical studies of sexual selection focused mainly on differences in male mating success, more recent work has focused on the differences in paternity share that may arise through sperm competition or cryptic female choice whenever females mate with multiple males. Thus, an integrative view of sexual selection needs to encompass processes that occur not only before copulation (pre-mating), but also during copulation (peri-mating), as well as after copulation (post-mating), all of which can generate differences in reproductive success. By encompassing mechanisms of sexual selection across all of these sequential reproductive stages this review takes an integrative approach to sexual selection in Tribolium flour beetles (Coleoptera: Tenebrionidae), a particularly well-studied and economically important model organism. Tribolium flour beetles colonize patchily distributed grain stores, and juvenile and adult stages share the same food resources. Adults are highly promiscuous and female reproduction is distributed across an adult lifespan lasting approximately 1 year. While Tribolium males produce an aggregation pheromone that attracts both sexes, there appears to be little pre-mating discrimination among potential mates by either sex. However, recent work has revealed several peri-mating and post-mating mechanisms that determine how offspring paternity is apportioned among a female's mates. During mating, Tribolium females reject spermatophore transfer and limit sperm numbers transferred by males with low phenotypic quality. Although there is some conflicting evidence, male copulatory leg-rubbing appears to be associated with overcoming female resistance to insemination and does not influence a male's subsequent paternity share. Evidence suggests that Tribolium beetles have several possible post-mating mechanisms that they may use to bias paternity. Male sperm precedence has been extensively studied in Tribolium spp. and the related Tenebrio molitor, and several factors influencing male paternity share among a female's progeny have been identified. These include oviposition time, inter-mating interval, male strain/genotype, the mating regimen of a male's mother, male starvation, and tapeworm infection. Females exert muscular control over sperm storage, although there is no evidence to date that females use this to differentiate among mates. Females could also influence offspring paternity by re-mating with additional males, and T. castaneum females more readily accept spermatophores when they are re-mating with more attractive males. Additional work is needed to examine the possible roles played by both male and female accessory gland products in determining male paternity share. Sexual selection during pre-mating episodes may be reinforced or counteracted by peri- and post-copulatory selection, and antagonistic coevolution between the sexes may be played out across reproductive stages. In Tribolium, males' olfactory attractiveness is positively correlated with both insemination success and paternity share, suggesting consistent selection across different reproductive stages. Similar studies across sequential reproductive stages are needed in other taxa to provide a more integrative view of sexual selection.

Ultrastructure of the seminal vesicle of Phlebotomus perniciosus Newstead (Diptera, Psychodidae)

The seminal vesicles of Phlebotomus perniciosus were investigated by light microscopy, confocal scanning laser microscopy and by scanning and transmission electron microscopy. They have a complex structure, and three different morphological compartments called A, B and C are distinguished on the basis of their position and fine structure. Compartment A is continuous with the vasa deferentia and consists of a cylindrical wall limiting a lumen in which the spermatozoa are stored. Compartment B is hemispherical and surrounds compartment A like a muff. Compartment C constitutes an external coat surrounding A and B. The epithelial cells of each compartment are characterized by morphologically different secretory granules. The ultrastructural features of these cells are described and their role in sandfly reproductive biology is discussed.

Cell type-specific gene expression in the Drosophila melanogaster male accessory gland

The accessory gland of the male Drosophila melanogaster plays a vital role in reproduction. This secretory organ synthesizes products that are transferred to the female and are necessary to elicit the proper physiological and behavioral responses in the female. The accessory gland is composed of two morphologically distinct secretory cell types, the main cells and the secondary cells. Previous studies identified some genes expressed in main cells or in all accessory gland cells. In this paper we use P-element mediated enhancer traps to examine gene expression in the accessory gland. We show that, in addition to genes expressed in main cells only or in all accessory gland secretory cells, there are genes expressed specifically in secondary cells. Each cell type is uniform in the expression of its genes. Our results demonstrate that the two cell types are not only morphologically distinct but also biochemically distinct. We also show that the two cell types differ in their regulation of gene expression in response to mating activity.

20-Hydroxyecdysone acts in the male pupa to commit accessory glands toward trehalase production in the adult mealworm beetle (Tenebrio molitor)

During postecdysial adult maturation, the bean-shaped accessory reproductive glands (BAGs) of adult male mealworm beetles produce increasing amounts of trehalase. In order to determine when the BAGs become competent to produce trehalase, we transplanted pupal BAGs into 0-day female adults. After 8 days, trehalase activity had increased in BAGs from 4- and 5-day pupae (at the time of the pupal ecdysteroid peak) but not in those from 1- and 2-day pupae (before the ecdysteroid peak). BAGs from 0- and 2-day pupae were exposed to 20-hydroxyecdysone in vitro before implantation into 0-day female adults. Increase in trehalase activity was dose dependent. Both dose (ED50, 5 x 10(-6) M) and exposure time (greater than 6 hr) of hormone required are greater for commitment than for acceleration of pupal cell cycling (T. Yaginuma, H. Kai, and G.M. Happ, 1988, Dev. Biol. 126, 173-181). Since trehalase activity increased markedly in isolated adult male abdomens, factors from the cephalic and thoracic centers are not required to sustain trehalase production in the adult BAGs.

Trehalase from the bean-shaped accessory glands and the spermatophore of the male mealworm beetle, Tenebrio molitor

In Tenebrio molitor, male adults transfer sperm to the female via a spermatophore or sperm sac. The spermatophore is formed from secretions of the bean-shaped accessory glands (BAGs) and the tubular accessory glands (TAGs) of the male beetle. Trehalase is found in the adult BAGs. During the pupal stage, the activity in the BAGs was very low. After adult ecdysis, the total activity increased 100-fold from 0 days to 6 days and reached maximum levels at 9 days. The specific activity increased 20-fold from the time of ecdysis to 6 days thereafter. In the 10 day adult, trehalase levels in testes, seminal vesicles, vas deferens, TAGs, or ejaculatory ducts, were lower by two orders of magnitude than in the BAGs. However, the specific activity in the spermatophore was similar to that in the BAGs. Trehalases in the BAGs and the spermatophores showed very similar properties (soluble, optimum pH of 5.75 and Km value of 5.4 mM for trehalose). Thus trehalase appears to be secreted from the BAGs and becomes incorporated into the spermatophores.

Ordered flow of secretion from accessory glands to specific layers of the spermatophore of mealworm beetles: demonstration with a monoclonal antibody

Monoclonal antibodies were produced against the secretory product of the bean-shaped accessory gland (BAG) of male mealworm beetles (Tenebrio molitor). Antibodies from one clone (PL 6.3) recognized a 9,600 dalton protein with a pI of 6.6 which was found in homogenates of the BAG. The PL 6.3 antigen was first detected on Western blots of BAG proteins from 2-day adults, and amounts increased for the next 6 days until reproductive maturation was achieved. The antibody also recognized a polypeptide with a molecular weight (mw) of about 5,000 daltons which we believe to be derived from the larger 9,600 dalton antigen. There are eight types of secretory cells in the BAG. By using light microscopic immunohistochemistry, we localized the antigens recognized by PL 6.3 in cell type 7 (intense staining) and cell type 5 (weak staining). Results from electron microscopic immunocytochemistry showed that antigen PL 6.3 was concentrated in the secretory granules characteristic of each of these two cell types and was absent in all other cell types. PL 6.3 antigens were traced from the BAG into its secretory product and then into the prespermatophoric mass in the ejaculatory duct. The antigen was not randomly mixed with other secretory products of the accessory glands. As it flowed from the BAG and into the ejaculatory duct, it remained in a coherent, precisely localized mass. Within the definitive spermatophore, the PL 6.3 antigen was concentrated in discrete layers of material that line the lumen.

Ecdysteroids accelerate mitoses in accessory glands of beetle pupae

During the 9-day pupal period of Tenebrio molitor (the mealworm beetle), the cells of the male accessory glands undergo divisions for 7 days. There are two maxima in the mitotic activity in the glands in vivo, one at 1 day and the other at 4 days. The latter peak coincides with the large surge of ecdysterone occurring in the pupal stage. By the use of in vitro culture techniques, it has been demonstrated that the first bout of mitosis in both glands proceeds in basal medium, while the second bout of mitosis requires a physiological level of ecdysterone. Ecdysone was less effective than ecdysterone. Sensitivity to ecdysterone did not change significantly between Day 1 and Day 4 of the pupal stage. The results are discussed in relation to the effects of ecdysterone on cell division in mesodermal and ectodermal derivatives.

Cell cycles in the male accessory glands of mealworm pupae

During the pupal stage of Tenebrio molitor, the accessory reproductive glands of males grow by cell division. Within the secretory epithelium of the bean-shaped accessory glands (BAGs), cell numbers triple. In the tubular accessory glands (TAGs), the increase is 14-fold. There are two mitotic maxima in each gland. The first maximum occurs at 1-2 days while the second is at 4-5 days. The second maximum coincides with the major ecdysteroid peak described by Delbecque et al. [Dev. Biol. 64, 11-30 (1978)]. Nuclei were isolated from TAGs during the pupal mitotic bouts and during mitotic inactivity in the adult. After Feulgen or propidium iodide staining, the DNA content of these nuclear populations was measured by absorption cytophotometry or by fluorescence flow cytometry, respectively. The proportion of cells in each phase of the cycle was calculated using an iterative model. After mitoses have ended in the late pupa, the cells were arrested in G2. [3H]Thymidine was injected into 1- and 4-day pupae to pulse-label cells of the TAGs. After allowing various periods from 4 to 60 hr for cells to progress through G2 to reach mitosis, fractions of labelled mitoses were determined by autoradiography. From the combined cytometric and autoradiographic data, the duration of each phase of the cell cycle was calculated assuming the population was in exponential growth. Cell cycles in 4-day pupal TAGs take 48 hr. G1, S, G2, and, M lasted 13, 14, 17, and 4 hr, respectively.