Wednesday, March 26, 2014

Direct fitness consequences of male mate size in Drosophila melanogaster

As previously mentioned back in the infancy of this blog, one of the tools we have in our lab is a sieve shaker system, which was originally designed in the Rice Lab at UCSB as a way to quickly sort phenotypic variation in body size in our Drosophila melanogaster populations. Recently (3 weeks ago), Pam (an incoming member of the Long Lab) set up a simple experiment as part of her fly-lab training. We were originally planning to look at differences in mating speed in single females placed into vials along a single male (from one of three treatments: "Large" - flies that had not passes through the holes in the 1420um sieve; "Medium" - flies that had passed through the holes in the 1262um sieve, not through the holes in the 1214um sieve; and "Small" - flies that had passed through the 1079um sieve). 

However, it transpired that the flies were not really co-operating (the subject of a post for another day), so instead we decided to change the assay (on the fly, so to speak) into an examination of how offspring production differed between the three treatments. We left the pairs of flies in the vials for 48h, then flipped the pairs into a second vial for a final 48h before being discarded. The vials were then incubated for 14 days, and then Pam & I counted the number of eclosed vials. Here are the results of this assay:
Right from the start, there is greater fecundity in the first 48h there was a difference in mean offspring production (ANOVA, F=4.5871, df=2,58, p= 0.014), with the lowest reproductive output coming from the Large male treatment, and the greatest from the small male treatment.
This pattern continued in the next 48 period. (ANOVA*, F=5.0757 , df=2,58, p= 0.009)
And not surprisingly, when data from the two 48h are combined (ANOVA*, F=8.0934 , df=2,58, p= 0.0007)

Now observations of this phenomenon is not new. There have been several studies that have shown this pattern including:

Pitnick, Scott, and Francisco García–González. "Harm to females increases with male body size in Drosophila melanogaster." Proceedings of the Royal Society of London. Series B: Biological Sciences 269.1502 (2002): 1821-1828.
Friberg, Urban, and Göran Arnqvist. "Fitness effects of female mate choice: preferred males are detrimental for Drosophila melanogaster females." Journal of evolutionary biology 16.5 (2003): 797-811.

..but I should note that in both of these (excellent, by the way) studies, male size was experimentally manipulated by varying the density of competing larvae, or the quality of their food. Our flies, which came from a large, outbred population (IV), had been cultured according to our standard laboratory protocol, so this shows that this phenomenon is something routinely encountered by female flies in the lab population, and not an artifact of the experiment. A the very least, it is a good snapshot of some of the selective pressures resulting for mate choice in this model species, and is a fun introduction to Pam to the world of behavioural ecology/evolutionary genetics.

Future studies will be focusing on more fine-scale variation of this phenomenon, and more importantly see how this aligns with observations on female mating preferences and/or the outcome of male-male competition, and even more importantly in the behaviour and fitness of the offspring. -TAFL
By the way, I used ggplot to make these graphs. Here is some sample code.

> jpeg("CUMULATIVE_eggs.jpg",quality=100)
> qplot(PAM$TREATMENT, PAM$CUMULATIVE, data=PAM, geom=c("boxplot"), 
+    fill=PAM$TREATMENT,
+    xlab="", ylab="Eggs laid over 96h")+ opts(legend.position = "none")
Created by Pretty R at

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