Gabriele Uhl

Securing paternity in spiders? A review on occurrence and effects of mating plugs and male genital mutilation

Low female mating frequencies often appear to be cases of direct male induction that can oppose female interests. Mating plugs are most obvious means leading to low degrees of multiple mating in females. In spiders, mating plugs are formed by a variety of amorphous materials, by the breakage of the male sperm transferring organ, or by the whole male that functions as a mating barrier. Our compilation of the available information on the presence of the various types of mating plugs suggests that plugs predominantly occur in entelegyne spiders. In this group, plugs do not interfere with oviposition since separate openings for insemination and oviposition are present. In contrast, mating plugs seem to be rare in haplogyne spiders that do not possess separate openings. The available experimental studies on the function of the different types of plugs suggest that plugs can be considered as male adaptations to avoid sperm competition. However, females in some cases were shown to have evolved means to prevent or control male manipulation or may selectively favour plug production in specific males, an aspect which has largely been neglected. In order to understand plug evolution and function we need to explore the morphological, behavioural and biochemical aspects involved and extend our approach to interactions between the sexes.

Sexual conflict over copulation duration in a cannibalistic spider

The evolutionary significance of sexual cannibalism depends on the timing of male death. Both female and male spiders possess paired genital organs and, while females can mate multiply, males of some species have limited mating rates because they can use each copulatory organ only once. By attacking a male after he has used one of his pedipalps but before he has used the other one, females may severely reduce male reproductive success. In the orb-web spider Argiope bruennichi, females attack every male during his first copulation and only 20% of virgin males survive such an attack. To measure costs and benefits associated with single versus double insertions for males, we created two double-mating treatments: we experimentally manipulated first males to use one or both of their pedipalps with a single female and then assessed paternity success of a second male (P2). Double insertions reduced P2 by increasing copulation duration rather than by increasing the number of insertions per se. When we separated the consumption of males experimentally from the act of killing, consumption had no effect on paternity. By counting sperm in used and unused pedipalps of males, we determined that sperm transfer is directly related to copula duration and that males are able to transfer 50% of their sperm load in each palp within 10xa0s of copulation. Our experiments revealed that sexual cannibalism after the first insertion is costly for males and we propose that these costs arise because females benefit from polyandry.

Fitness consequences of sexual cannibalism in female Argiope bruennichi

The evolution of sexual cannibalism as the most extreme form of nuptial feeding is still poorly understood. Although increasing evidence suggests that female aggressiveness is related to other aspects of foraging behaviour, it is not clear whether the nutritional value of a male is sufficient to provide an adaptive significance for sexual cannibalism. A widely cited though rarely tested explanation is based on a paternal investment model, and predicts that consumption of a male results in increased female fecundity. The available evidence is either correlational or restricted to species with relatively large and potentially nutritious males, and different studies have come to different conclusions. Here we present a test of the paternal investment hypothesis using the very cannibalistic and highly size-dimorphic spider Argiope bruennichi. After a preset schedule, we had females consume none, one or two males independent of the females cannibalistic behaviour. Consumption of male bodies did not result in any detectable fitness benefit for the female: neither the number of clutches, nor clutch size or hatching success were affected by consumption of males. The frequency of cannibalism was around 80%, independent of the female mating status. We did not observe male complicity, but cannibalism was associated with prolonged copulation. This suggests a sexually selected benefit of cannibalism for males. We conclude that the paternal investment hypothesis does not explain the existence of sexual cannibalism in A. bruennichi and probably not in other spider species with a pronounced sexual size dimorphism.

Potential and limitations of X-Ray micro-computed tomography in arthropod neuroanatomy: A methodological and comparative survey

Classical histology or immunohistochemistry combined with fluorescence or confocal laser scanning microscopy are common techniques in arthropod neuroanatomy, and these methods often require time‐consuming and difficult dissections and sample preparations. Moreover, these methods are prone to artifacts due to compression and distortion of tissues, which often result in information loss and especially affect the spatial relationships of the examined parts of the nervous system in their natural anatomical context. Noninvasive approaches such as X‐ray micro‐computed tomography (micro‐CT) can overcome such limitations and have been shown to be a valuable tool for understanding and visualizing internal anatomy and structural complexity. Nevertheless, knowledge about the potential of this method for analyzing the anatomy and organization of nervous systems, especially of taxa with smaller body size (e.g., many arthropods), is limited. This study set out to analyze the brains of selected arthropods with micro‐CT, and to compare these results with available histological and immunohistochemical data. Specifically, we explored the influence of different sample preparation procedures. Our study shows that micro‐CT is highly suitable for analyzing arthropod neuroarchitecture in situ and allows specific neuropils to be distinguished within the brain to extract quantitative data such as neuropil volumes. Moreover, data acquisition is considerably faster compared with many classical histological techniques. Thus, we conclude that micro‐CT is highly suitable for targeting neuroanatomy, as it reduces the risk of artifacts and is faster than classical techniques. J. Comp. Neurol. 523:1281–1295, 2015.

The male genital system of the cellar spider Pholcus phalangioides (Fuesslin, 1775) (Pholcidae, Araneae): development of spermatozoa and seminal secretion

BackgroundMost arthropods pass through several molting stages (instars) before reaching sexual maturity. In spiders, very little is known about the male genital system, its development and seminal secretions. For example, it is unknown whether spermatozoa exist prior to-, or only after the final molt. Likewise, it is unclear whether sperm are produced throughout male adulthood or only once in a lifetime, as is whether seminal secretions contain factors capable of manipulating female behavior. In order to shed light on these aspects of the reproductive biology of spiders, we investigated the male genital system of the common cellar spider Pholcus phalangioides, with special emphasis on its development and seminal secretions.ResultsTestes already display all stages of spermatogenesis in subadult males (about four weeks before the final molt). Their vasa deferentia possess proximally a very voluminous lumen containing dense seminal fluid and few spermatozoa, whereas the distal part is seemingly devoid of contents. Spermatoza of P. phalangioides are typical cleistospermia with individual secretion sheaths. In male stages approximately two weeks prior to the final molt, the lumina of the testes are wider and filled with a dense secretion. The wide, proximal portion of the vasa deferentia is filled with secretion and a large number of spermatozoa, and the narrow distal part also contains secretion. In adult males, the wide lumina of the testes are packed with spermatozoa and secretions. The latter are produced by the somatic cells that bear microvilli and contain many vesicles. The lumina of the vasa deferentia are narrow and filled with spermatozoa and secretions. We could identify a dense matrix of secretion consisting of mucosubstances and at least three types of secretion droplets, likely consisting of proteinaceous substances.ConclusionThis study reveals that spermatogenesis begins weeks before maturity and takes place continuously in the long-lived males of P. phalangioides. Possible functions of the various types of secretion in the seminal fluid and previously investigated female secretions are discussed in the light of sexual selection.

Two distinctly different sperm storage organs in female Dysdera erythrina (Araneae: Dysderidae)

The haplogyne spider D. erythrina possesses two distinctly different sperm storage organs: a bilobed anterior spermatheca and a large, sac-like posterior diverticulum. The glandular equipment of both storage types is markedly different: the glandular tissue of the spermatheca is composed of complicated glandular units comprising a cuticular ductule and three canal cells (class 3 cells) whereas the glandular tissue of the posterior diverticulum is composed of simple gland cells that discharge their product through the cuticle (class 1 cells). Thus, the glandular products produced differ, leading to different storage conditions for the spermatozoa from copulation to egg laying. It is suggested that multiple organ types have evolved to facilitate specialization in short-term and long-term storage and to allow (posterior diverticulum) or prevent (spermatheca) males from accessing previously stored sperm.

The sex pheromone of the wasp spider Argiope bruennichi.

Current biochemical research on spiders focuses on the two characteristic traits common to spiders, namely silk and toxins. Nevertheless, chemical communication over longer distances is also vital: potential mates that are solitary and predatory need to find members of the same species (conspecifics) over large distances during the reproductive season. The production of a volatile signal that helps to attract mating partners is vital, but to date only few of the respective pheromones are known (see the following Communication). Herein we report the identification of the first pheromone from an orb weaver (Araneidae), namely the wasp spider, Argiope bruennichi, and show for the first time that a spider pheromone can be used to trap spiders in the field. Orb weavers are one of most successful spider families worldwide, with over 2600 known species. Early experiments showed that Cyrtophora cicatrosa emits a volatile pheromone that attracts males and induces courtship in higher concentrations. Cages with females of Araneus trifolium and Argiope trifasciata were able to attract conspecifics. The latter also attracted Argiope aurantia males, suggesting the presence of a common sex pheromone in both species. Sex pheromones that attract conspecific males over a distance to the web of a partner has so far only been reported from the desert spider Agelenopsis aperta (Agelenidae) that uses 8methyl-2-nonanone. We investigated the pheromone system of the very characteristic wasp spider Argiope bruennichi, which lives on meadows in the Mediterranean but occurs now also in the temperate zone of Middle Europe. When adult, the large females build webs in the grass, whilst the much smaller males wander through the meadow to find a mate, probably guided by a female pheromone. Herein, we report on the identification, synthesis, and activity of this pheromone in the field. Headspace sampling of odors emitted by individual female spiders was performed with activated charcoal traps using glass chambers containing the spiders in three developmental and reproductive states (subadult, virgin, and mated). The adsorbent was extracted with dichloromethane and the extracts analyzed by GC-MS. Only the extract of virgin adult spiders contained a compound A that was absent from subadult or mated females (Figure 1).

Copulation patterns in the golden orb-web spider Nephila madagascariensis

A consequence of multiple mating by females can be that the sperm of two or more males directly compete for the fertilisation of ova inside the female reproductive tract. Selection through sperm-competition favours males that protect their sperm against that of rivals and strategically allocate their sperm, e.g., according to the mating status of the female and the morphology of the spermatheca. In the majority of spiders, we encounter the otherwise unusual situation that females possess two independent insemination ducts, both ending in their own sperm storage organ, the spermatheca. Males have paired mating organs, but generally can only fill one spermatheca at a time. We investigated whether males of the African golden orb-web spider Nephila madagascariensis can prevent rival males from mating into the same spermatheca and whether the mating status of the female and/or the spermatheca causes differences in male mating behaviour. There was no significant difference in the duration of copulations into unused spermathecae of virgin and mated females. We found that copulations into previously inseminated spermathecae were generally possible, but shorter than copulations into the unused side of mated females or with virgins. Thus, male N. madagascariensis may have an advantage when they mate with virgins, but cannot prevent future males from mating. However, in rare instances, parts of the male genitals can completely obstruct a female genital opening.

Mate choice in males with one-shot genitalia: limited importance of female fecundity

Male mate choice is expected in species in which future mating opportunities of males are strongly diminished after their first copulation and mate quality is variable unless the costs of choice exceed the benefits. Males of the spider Argiope bruennichi are mostly monogynous; they damage their paired one-shot genitalia during copulation and fall victim to sexual cannibalism. Even males that are not cannibalized can achieve a maximum of two matings. Such a high mating effort implies that a male’s reproductive success can be increased by selecting the best possible mate. Mate quality may depend on female mating status and/or fecundity. Males should be selected to prefer virgin females over mated ones since previously used female genital ducts can be blocked by genital mating plugs. Fecundity is a common approximation of female quality and in invertebrates is generally correlated with body size and mass. Argiope bruennichi females vary in body size and fecundity, providing a basis for male selectivity. By creating binary choice situations in field and laboratory experiments in which search costs were largely removed, we tested whether males show mate choice. There was weak evidence for a male preference based on female phenotype. However, virgin females were significantly preferred over mated females even if the virgin had a less fecund phenotype. We conclude that selective benefits of mating first with a female override potential benefits of choice based on female fecundity, leading to competition for virgin females.

Sperm storage mediated by cryptic female choice for nuptial gifts

Polyandrous females are expected to discriminate among males through postcopulatory cryptic mate choice. Yet, there is surprisingly little unequivocal evidence for female-mediated cryptic sperm choice. In species in which nuptial gifts facilitate mating, females may gain indirect benefits through preferential storage of sperm from gift-giving males if the gift signals male quality. We tested this hypothesis in the spider Pisaura mirabilis by quantifying the number of sperm stored in response to copulation with males with or without a nuptial gift, while experimentally controlling copulation duration. We further assessed the effect of gift presence and copulation duration on egg-hatching success in matings with uninterrupted copulations with gift-giving males. We show that females mated to gift-giving males stored more sperm and experienced 17% higher egg-hatching success, compared with those mated to no-gift males, despite matched copulation durations. Uninterrupted copulations resulted in both increased sperm storage and egg-hatching success. Our study confirms the prediction that the nuptial gift as a male signal is under positive sexual selection by females through cryptic sperm storage. In addition, the gift facilitates longer copulations and increased sperm transfer providing two different types of advantage to gift-giving in males.