I really like this song! For those who do not know (shame on you!), it is John Lee Hoocker, one of the best bluesmen ever. I could keep on talking about blues, but my guess is it is far better to let you listen to it 🙂 So, let me go into something close to this song: turning Drosophila males’ heads. Indeed, a study published in the very last BMC Genomics shows that fruitfly males are under the spell: gene expression profiles in their heads are altered because of mating. Boom boom boom…
Behaviour is a complex process resulting from the integration of internal (genetic make-up and nervous system function) and external (environmental) information. The latter includes social interactions. Those are known to alter morphology and gene expression, but the mechanisms mediating these modifications are unclear. Thus, the authors chose the model organism Drosophila melanogaster for which a great amount of methods is available and a considerable knowledge on behavioural physiology and genetics is accumulated.
The authors then combine microarray techniques with behavioral assays in order to determine which genes, regulating or regulated by behaviour, have modified expression profile. Previous studies have shown that gene expression in males is altered at the whole-body level during courtship. More specifically, the current study focuses on changes occurring in the male head as a result of mating « since these changes likely affect function of the nervous system and other reproductively important tissues to promote reproductive success », explain the authors.
Here they used male flies that mated with females and males which were not presented to a female as a control. So, the first result is that there are 47 mating-responsive genes coming out significant: 2 hours after mating with a female, males significantly up-regulated 25 genes and down-regulated 22 others. As the authors precise it, « such changes are not likely to be activity-dependent since control males had locomotor levels similar to males that courted females ».
Furthermore, a hypothesis was formulated regarding where these mating-responsive genes are expressed given that the idea here is to survey for genes potentially functioning in reproduction by regulating nervous system signaling. What was found is that many of these differentially expressed genes are expressed in the head, but not in the brain: lots of them were found to be expressed in the fat body (this is the adipose tissue lining the head cavity which surrounds the brain). The fat body has already been reported to be implicated in courtship behaviour. Nice picture here: mating-induced changes in gene expression occur in the fat body.
At the end, the authors checked how important some juvenile hormone esterases are for male reproductive behaviour. The hypothesis here was that if a gene is up-regulated after mating, that gene likely affects some aspect of reproductive behaviour. To test this, authors assayed the courtship index, CI (which is the percentage of time a male spends courting a female) of candidate mating-responsive gene mutants. In brief, homozygous mutant males had significantly reduced CI values compared to heterozygous or wild-type controls. The courtship latency (ie, the time to initiation of courtship) was significantly increased in homozygous mutants while mating duration was unaffected. This increased mating latency was independent on the mating trial (1st, 2nd, 3rd) and no detectable fertility effects were observed.
Here come the conclusions then. In comparison to a previous study where whole males were used for monitoring of transcriptional changes after 5 minutes of courting (without mating), there was only one gene overlapping. Which is somehow perfectly normal given that nothing is comparable: neither the time points (5 min courting vs. 2 hours post-mating), nor the tissues (whole bodies vs. heads), nor behaviours (courting vs. courting + mating), nor the analysis procedures. What also comes up from this study is that the majority of mating-responsive genes are expressed in the fat body which is consistent with previous studies. Therefore, the brain is not the only tissue responding to or regulating post-mating behaviour: fat body plays an important role as well.
My conclusion is: I should say I was really disappointed. To be honest, I found that the interesting stuff in this paper was following a very strong gradient from « wow, cool » towards « pfff, boring »: the title and the introduction were attractive and interesting, the results were somehow not that exciting and the discussion (the part I search for the most in a paper) is just crap. 4 pages of speculation on juvenile hormone esterases, some perfectly recycled blahblah on the fact that the brain is not the only organ involved in reproductive behaviour,… But the icing on the cake is the very last sentence:
We predict that mating-responsive genes also function in other aspects of reproduction and behaviour; therefore, we propose this genome-wide approach as powerful tool for determining the genetic pathways and intracellular processes regulating reproduction, both at the behavioural and physiological levels.
No further comment…
Ellis LL, & Carney GE (2010). Mating alters gene expression patterns in Drosophila melanogaster male heads. BMC genomics, 11 (1) PMID: 20937114