Amy Brunton Martin talks about her new paper, Orchid sexual deceit affects pollinator sperm transfer, which shows the costs of being fooled for an orchid pollinator, and highlights the importance of considering both sides of a relationship when looking at deceptive interactions in nature.
About the paper
Some flowers, orchids in particular, release scents that match the sex pheromones of female insects, thus males are tricked into perceiving flowers as females. Attracted males mate with these flowers and sometimes even waste their sperm during these trysts. Known as sexual deception, this technique is used by ¬400 species of orchids for pollination. The Australasian tongue orchids, Cryptostylis, are excellent examples of sexual deceivers – often causing their ‘orchid dupe wasp’ pollinator, Lissopimpla excelsa to ejaculate and waste their sperm.
We asked if sperm loss was costly, and hence affect whether this relationship persists over evolutionary time. If males can’t regenerate sperm or if they waste large quantities of sperm on orchids, then females of the pollinator species may never receive sperm, affecting their reproduction.
We compared the sperm stock of males that were either allowed to mate with an orchid, or prevented from mating so we could see how much sperm males use in each mating. To prevent males from mating (and prevent too many males mating with an orchid at once!) we kept our orchid-bait in a mesh tent during experiments and waited for it to lure in male insects. We then caught the males and dissected out their seminal vesicles (sperm storage organs) to count their sperm. We also counted the amount of sperm males deposited on orchids. To see if long-term interactions with orchids affected male sperm use, we repeated this experiment at sites with natural populations of orchids and places without orchids.
We found that males that mated with orchids had significantly less sperm than males that did not. This suggests that males can run out of sperm, even if only temporarily. We also found that males from sites with orchids deposited significantly less sperm on orchids, and that males decreased their deposit size as their sperm stock depleted. These changes in sperm use might be due to one of two things. Males at sites with orchids may be wary of deceptive plants, and so spend less sperm on an orchid. Alternatively, when orchids are present, this could fool males into behaving as if there is a very high availability of females in their local area – and so, to maximise the number of successful mating events deploy their sperm in smaller bundles.
About the research
One thing that was reasonably straight forward, though, was getting the males in to mate with the orchids. They would appear very quickly (within 15 minutes) – if they didn’t, I knew they weren’t out flying that day.
This research was great to carry out – but it had some pitfalls. Counting sperm can take a long time – I spent hours looking down a microscope counting bright blue dots of sperm. Luckily, there are many good podcasts available (I’d recommend In Situ Science if you’re interested in weird animal stuff, or NADDPOD if you like great storytelling). One thing that was reasonably straight forward, though, was getting the males in to mate with the orchids. They would appear very quickly (within 15 minutes) – if they didn’t, I knew they weren’t out flying that day. That meant I could have some time off to enjoy the sights of Sydney! However, getting only one wasp at a time was tricky – so I had to conduct all my experiments in a small tent. Otherwise, I would have too many and not know who the sperm came from!
Sperm use is important for most animals as it can change how populations reproduce and survive. In the case of these orchids — females that miss out on sperm due to orchid interference might not suffer as female wasps are haplodiploid (where sons come from unfertilised eggs and daughters are produced from fertilised eggs). If females have sperm, they can produce sons or daughters – if they don’t, they can still have sons. Most pollinators of sexually deceptive orchids are haplodiploid – does this ability to reproduce asexually confer pollinators some advantage against sexual deception? Is it advantageous in other costly relationships?
This research hopefully highlights the importance of considering both sides of a relationship when looking at deceptive (or costly) interactions in nature. It also shows that males probably are important for populations – something not usually thought about, as it’s often assumed, they have an unlimited supply of sperm!
About the author
I’m lucky enough to have a mother for an ecologist – so I owe my interest in the natural world to her. I grew up being taken along on field trips to offshore islands all around New Zealand – and being quizzed on our native plants and birds! As I got older, I found interest in many things (dungeons and dragons is a great passion of mine!) – but always came back to science and keeping a curious mind. That’s something that I tried to remember when I found myself in a small room with a microscope for days; knee deep in swamps (with no wasps in sight!) in Northland; or in sweltering 40oC heat in Sydney. Having a question and then being able to go out and find the answer is a wonderful privilege.
I have just finished my PhD, and I’m currently working on a number of different things – I’m trying to get the results of my PhD out into the world (more sperm and orchid work!), but also modelling bird vision, and collaborating with other students in my lab. I’m looking for a permanent position, now – hopefully working on more intriguing systems!