Why females mate once: from genes to populations

Lead Research Organisation: University of Liverpool
Department Name: Sch of Biological Sciences

Abstract

Some female animals mate once in their life while others mate with many males each day. This results in enormous differences between species in everything from their physiology and behaviour, to how their social systems are arranged. Many animals are highly adapted to a system where females mate frequently. A male will generally have fewer offspring if a female he mates with goes on to mate with another male, who will father some of her offspring. This evolutionary pressure on males has caused the evolution of male traits that reduce female remating rates. The establishment of harems in red deer, elephant seals and gorillas are well known examples. Male honeybees genitalia burst inside the female in an effort to block her reproductive tract and prevent her mating with other males. Females in turn have often evolved traits that allow them to avoid control by males, and remate with males of their choosing. Finding the genes underlying female remating would be a big step forward in understanding all this variation. I will search for these genes using the fruit fly Drosophila subobscura. Female flies of this species from Greece remate, whereas those from England do not. Fortunately, the flies are reproductively compatible, so I can crossbreed flies from Greece and England over several generations. This will mix the genes they carry each generation, resulting in many lines of flies that contain a random mix of Greek and English DNA. Then I can test these flies for willingness to remate. Some will be willing to remate despite inheriting only a small amount of DNA from the Greek (willing to remate) population. This will tell me that genes important for willingness to remate must be found in that section of DNA. I can work out which sections of DNA are Greek and which are English by looking at tiny differences in the DNA of flies from the two populations at regular intervals along each chromosome. I will be able to determine how many areas of the genome are important for remating, and will link this to genes examined in closely related species that scientists think may be important in controlling remating. If we can understand which genes cause remating, this will help us understand the mechanisms and consequences better. Moreover, if we can use this knowledge to develop ways to prevent remating in pest species, such as mosquitoes and medflies, many insect control techniques will become much more effective. Remating also seems to be very important in preventing the spread of selfish genes that distort sex ratios. In Tunisian populations of this fly about 20% of flies carry a driving X chromosome called SRS. Normal X chromosomes are passed on to half a male's offspring, while the other half inherits his Y chromosome. But when males carry the SRS chromosome all their Y bearing sperm die and all their offspring inherit the SRS X chromosome. This allows the SRS chromosome to spread as it is passed on to more offspring that the normal X, but it also causes male carriers to only have daughters, and to produce less sperm than normal males. This can cause populations to mostly consist of females, and potentially could wipe entire populations out due to a total lack of males. Work in related species has shown that if females mate with multiple males the small amounts of sperm produced by carrier males is usually swamped by the large amounts of sperm transferred by normal males, and the driving X cannot spread. But in D. subobscura, SRS is only found in the Southern populations where females remate, and is never found in the Northern populations where females mate once. I will investigate why this happens by setting up many small laboratory populations of Greek and English flies, with SRS at 20%, set up at different temperatures. I will track the frequency of SRS over many generations, and will be able to determine the conditions under which SRS can spread, and why it is not found in the Northern single mating populations.

Publications

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Description Some female animals mate once in their life while others mate with many males each day. This results in enormous differences between species in everything from their physiology and behaviour, to how their social systems are arranged. However, we have limited understanding of why species and individuals vary so much in mating rate.
I first attempted to examine these questions in the fruit fly Drosophila subobscura. Females of this species in Greece were thought to remate often, whereas those from the UK only mate once in their lives. However, my detailed investigation of this showed that the Greek flies do not remate- all females of this species only mate once (Fisher et al., 2012, BMC Evolutionary Biology). Hence I shifted my study to a related species, Drosophila pseudoobscura. In this species I found that females from populations in the southern USA rarely remate, whereas those from the northern USA remate often (Price et al., 2014, Proc R Soc B). A major topic of my research since then is to investigate why this happens- what are the genes that cause this difference in behaviour between these populations. I have been using genome sequencing and proteomics to find out, but the results are not yet ready for publication.
The next strand of my research has been to investigate the consequences of variation in female remating rate. I found that in the species where females only mate once (D. subobscura), males fight intensely over access to females, and males exposed to rival males are so damaged by the conflict they are often barely able to mate at all, and die very young (Lize et al 2014, Proc R Soc B). However, in the species where female do remate (D. pseudoobscura) males are more measured in their response to rivals, increasing the amount of sperm they transfer to females after they have encountered rivals, to ensure they father more of a female's offspring (Price et al 2012 J Insect Physiology). I also found some evidence that differences in how males respond to rivals is genetically determined (Bretman et al 2014 PlosOne).
Rates of remating have been reported for a wide variety of organisms, from fish to butterflies, using the standard technique of microsatellites, a type of genetic fingerprinting. I reviewed most of this data, finding that females in 89% of species show remating (Taylor et al, 2014, Trends Ecol Evol). However, this means that females from 11% of species never remate, and even within species that do remate, we found that on average about 10% of individual females were found to have mated only once. This was the largest scale review of female remating, and this high rate of females refusing to remate was unexpected, and is a mystery yet to be explained.
Exploitation Route The meiotic drive system in D. subobscura studied in this project has potential for use in developing SIT and other pest control techniques. SIT companies may be interested in it.
Sectors Agriculture

Food and Drink

 
Description This is blue skies research, and much of it has only been published very recently. It will take some time for it to have an economic impact. However, it has been used in education and the promotion of science.
First Year Of Impact 2014
Sector Education
Impact Types Cultural

 
Description Gave a talk at a 6th form college (Brighton) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact 60 students attended two talks about sexual selection and evolution and what a biology degree is like, which sparked questions and discussion and informed students about how to continue to study biology in higher education

Several students asked detailed questions about how to do a degree in biology, and many students were enthused about biology
Year(s) Of Engagement Activity 2014
 
Description Outreach experiments for students from low university participation schools 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Each year we run a summer school at university for talented pupils from schools that have very low participation rates at university, to encourage university enrollment. Each summer ~60 students run experiments with us on Drosophila. Feedback from the event is consistently very good, with many students saying it is their favourite part of the summer school.
Year(s) Of Engagement Activity 2013,2014,2015,2016,2017,2018
 
Description Sixth form college talk in Crosby 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Gave a talk on sexual conflict and evolution to 40 6th form students, which sparked questions and discussions

Several students said the talk made them want to continue in education, and ask questions about how best to do that
Year(s) Of Engagement Activity 2013
 
Description Talk at a 6th form school (Liverpool) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Gave a talk to biology students at the Belvedere Academy, Liverpool, presenting my work and the field of sexual selection generally.

The students really liked the talk and got really excited about biology
Year(s) Of Engagement Activity 2013,2014
 
Description Talk at secondary school (Liverpool) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact Gave a talk about sexual conflict and evolution, to GCSE students, sparking questions and discussion, and enthusing the students about biology

Many of the students got very excited about biology
Year(s) Of Engagement Activity 2013
 
Description Teaser experiment for students visiting Liverpool University 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Talented students from schools that send very few students to university came to visit our institution, with the aim of encouraging them to apply to university. I ran a short experiment with the students to give them a taste of science, and behavioural ecology.

Many of the students got really excited about the experiment and about doing real research for the first time in their lives
Year(s) Of Engagement Activity 2013,2014