Periodic Reporting for period 2 - Widow Spider Mating (Immature mating as a novel tactic of an invasive widow spider)
Reporting period: 2022-02-10 to 2023-02-09
Mating with immature females was previously considered impossible, as by definition only adult individuals were regarded sexually mature, possessing developed reproductive organs enabling them to copulate and reproduce. Nevertheless, in several widow species (genus Latrodectus) immature (subadult) females engage in copulation and, once reaching maturity, produce viable offspring. Although widow spider females are notoriously known for their cannibalistic tendencies, males are not attacked by subadult females. Apart from lack of cannibalism, immature mating is characterized by much-reduced courtship and, due to the lack of cannibalism, a higher number of insertions during copulation. However, other consequences of immature mating are not known. Within this project, we aimed to evaluate costs and benefits of immature mating in the brown widow spider L. geometricus. We explore the underlying behavioural and morphological mechanisms and its impact on male and female reproductive success in comparison to suicidal mating with adult females to identify trade-offs of both mating tactics and to help us understand how they evolved. This project mainly focuses on basic, fundamental research of behavioural and morphological aspects of immature mating. Considering the invasive nature and medical importance of the widow spiders, my investigation of this flexible mating system may help to lay groundwork for developing a non-toxic pest control tool based on mate attraction in future.
The outgoing phase was realized in the lab of the leading expert on mating in widow spiders, Prof. Maydianne Andrade at the University of Toronto. We examined male mate choice in the brown widow spider, L. geometricus and revealed that males prefer adult females (Sentenská et al. 2022). Further, we examined courtship behaviours towards adults and subadults and cues triggering them. To determine which types of cues trigger different courtship components, we swapped adults and subadults between webs. We showed that contact with webs and bodies of adults triggers majority of the behaviours, only vibrational signals occur regardless of the female developmental stage (Sentenská et al. 2022).
We further investigated potential costs of immature mating in terms of higher re-mating rates of subadults, a behaviour that would decrease paternity of the first male to mate. Males typically engage in self-sacrificial behaviour when mating with adults as they somersault into their mouthparts during copulation, triggering female cannibalism. In the congener L. hasselti, this behaviour increases copulation duration and lowers re-mating probability of mated females. Since males do not perform the somersault with subadults, we expected their re-mating rate to be significantly higher than in adults that experienced self-sacrifice. Our experiments, however, revealed the opposite - females who mated as subadults were significantly less likely to re-mate again in their adult stage than females who mated first as adults and experienced self-sacrifice (Sentenská et al. 2023). Additionally, unlike in its congener, male self-sacrifice in L. geometricus did not affect copulation duration (Sentenská et al. 2023). Consequently, despite the apparent similarities in self-sacrificial behaviour, its function obviously differs between the two self-sacrificial species.
We also examined female behaviour, namely their pre- and post-copulatory choice. We expected adults to have higher risk of delays to reproduction than subadults, yet, we observed that females of both stages mate indiscriminately during their first mating (Sentenská et al. 2023). This observation is in concert with the ‘wallflower’ hypothesis that proposes females mating indiscriminately to avoid reproductive delays. Interestingly, adult females usually allowed only a single insertion during their first mating, leaving one of their paired sperm storage organs empty (Sentenská et al. 2023). Additionally, both adult- and subadult-mated females re-mated with higher probability with males larger or heavier than their first mating partner, indicating that both female stages are 'trading up' through re-mating with higher quality mates (Sentenská et al. 2023). Our results show that the ‘wallflower’ effect and ‘trading up’ tactics can be utilized at different life stages by females of L. geometricus, allowing them to employ choice even if rejecting males is costly.
During the return phase realized in the lab of Prof. Gabriele Uhl at the University of Greifswald, Germany, we explored the morphological aspect of male self-sacrifice. Prof. Uhl is a leading expert on mating in various spider species and, in her research, she combines behavioural experiments with morphological analyses. In her lab, I explored the interplay between male and female genitalia of L. geometricus during copulation. Specifically, we examined whether the genital coupling differs between normal 'self-sacrificial' mating and immature mating. With the help of state-of-the-art methods (in-copula fixation, microtomography and 3D reconstruction) we revealed that despite different mating positions there is no difference in genital coupling between adult and immature mating (Sentenská, in prep.).
Through another set of experiments we explored paternity assurance in form of plugging in L. geometricus. Males of widow spider break off the tips of their paired copulatory organs, thereby plugging the female genitalia and hindering female re-mating. Although males mating with subadults typically achieve two insertion and so potentially block both of female spermathecae, the subadult genitalia might not be hardened enough to hold the plugs. The plug placement has been investigated via dissection and examination of genitalia of mated males and females. Contrary to our expectations, the plugging success was higher in subadults as mating plugs were found in both of their spermathecae with higher probability compared to adults (Sentenská et al. 2021).
Our results were presented on two online and four in-person conferences. So far, three papers has been published in high-ranking peer reviewed journals as Open Access and one more paper is being currently finalized and will be submitted soon.
We further investigated potential costs of immature mating in terms of higher re-mating rates of subadults, a behaviour that would decrease paternity of the first male to mate. Males typically engage in self-sacrificial behaviour when mating with adults as they somersault into their mouthparts during copulation, triggering female cannibalism. In the congener L. hasselti, this behaviour increases copulation duration and lowers re-mating probability of mated females. Since males do not perform the somersault with subadults, we expected their re-mating rate to be significantly higher than in adults that experienced self-sacrifice. Our experiments, however, revealed the opposite - females who mated as subadults were significantly less likely to re-mate again in their adult stage than females who mated first as adults and experienced self-sacrifice (Sentenská et al. 2023). Additionally, unlike in its congener, male self-sacrifice in L. geometricus did not affect copulation duration (Sentenská et al. 2023). Consequently, despite the apparent similarities in self-sacrificial behaviour, its function obviously differs between the two self-sacrificial species.
We also examined female behaviour, namely their pre- and post-copulatory choice. We expected adults to have higher risk of delays to reproduction than subadults, yet, we observed that females of both stages mate indiscriminately during their first mating (Sentenská et al. 2023). This observation is in concert with the ‘wallflower’ hypothesis that proposes females mating indiscriminately to avoid reproductive delays. Interestingly, adult females usually allowed only a single insertion during their first mating, leaving one of their paired sperm storage organs empty (Sentenská et al. 2023). Additionally, both adult- and subadult-mated females re-mated with higher probability with males larger or heavier than their first mating partner, indicating that both female stages are 'trading up' through re-mating with higher quality mates (Sentenská et al. 2023). Our results show that the ‘wallflower’ effect and ‘trading up’ tactics can be utilized at different life stages by females of L. geometricus, allowing them to employ choice even if rejecting males is costly.
During the return phase realized in the lab of Prof. Gabriele Uhl at the University of Greifswald, Germany, we explored the morphological aspect of male self-sacrifice. Prof. Uhl is a leading expert on mating in various spider species and, in her research, she combines behavioural experiments with morphological analyses. In her lab, I explored the interplay between male and female genitalia of L. geometricus during copulation. Specifically, we examined whether the genital coupling differs between normal 'self-sacrificial' mating and immature mating. With the help of state-of-the-art methods (in-copula fixation, microtomography and 3D reconstruction) we revealed that despite different mating positions there is no difference in genital coupling between adult and immature mating (Sentenská, in prep.).
Through another set of experiments we explored paternity assurance in form of plugging in L. geometricus. Males of widow spider break off the tips of their paired copulatory organs, thereby plugging the female genitalia and hindering female re-mating. Although males mating with subadults typically achieve two insertion and so potentially block both of female spermathecae, the subadult genitalia might not be hardened enough to hold the plugs. The plug placement has been investigated via dissection and examination of genitalia of mated males and females. Contrary to our expectations, the plugging success was higher in subadults as mating plugs were found in both of their spermathecae with higher probability compared to adults (Sentenská et al. 2021).
Our results were presented on two online and four in-person conferences. So far, three papers has been published in high-ranking peer reviewed journals as Open Access and one more paper is being currently finalized and will be submitted soon.
Our investigation revealed that brown widow males prefer to approach and court adult over subadult females. The shorter courtship and the lack of self-sacrifice during immature mating were previously considered to be benefits a male experiences when mating with subadults and therefore a preference of subadult females was expected. However, they rather indicate a lower investment into immature mating and further support the consistent male preference of adult females. Interestingly, our investigation revealed another benefits of immature mating - despite the lower investment, males mating with immature females gain higher mating success in terms of higher number of insertions and higher plugging success while the risk of female re-mating is lower than in adults. Additionally, the copulatory somersault enabling male self-sacrifice does not alter genital coupling and is obviously not necessary for successful sperm transfer. Therefore, it remains puzzling why males engage in this behaviour and why do they consistently prefer adults over subadults.