Variation in the amount and activity of esterase 6 in a natural population of Drosophila melanogaster

Nucleotide polymorphism in the Est6 promoter, which is widespread in derived populations of Drosophila melanogaster, changes the level of Esterase 6 expressed in the male ejaculatory duct

Previous analysis of an Australian population of D. melanogaster revealed two predominant Est6 promoter haplotypes, P1 and P7. These haplotypes, which differ at 14 sites over a 325-bp region, are associated with a 15-20% difference in male EST6 activity. Here we show that the P1/P7 sequence difference causes the male activity variation by recreating the activity difference among >60 independently transformed lines containing representative P1 or P7 promoter alleles fused to an identical Est6 coding region. Furthermore we find that the whole fly difference reflects about a twofold difference in EST6 activity in the anterior sperm ejaculatory duct. EST6 activity variation in this tissue is known to affect reproductive fitness. Using a combination of RFLP analysis and DNA sequencing, we show that P1 and P7 are predominant in six populations from America, Asia, and Australia, albeit less frequent in a population from the presumptively ancestral east African range of the species. The sequence data show significant departures from neutral expectations for the derived American and Australian populations but not the presumptively ancestral Zimbabwean population. Thus the P1/P7 difference could be a major source of adaptively significant EST6 activity variation through much of the now cosmopolitan range of D. melanogaster.

Causes and consequences of esterase 6 enzyme activity variation in pre-adult Drosophila melanogaster

We report heritable threefold differences in both larval and pupal esterase 6 activity among 17 isoallelic lines of D. melanogaster extracted from a natural population. The activity differences in the two stages are only weakly correlated with each other or with previously determined values for esterase 6 activity in adults of these lines. The pre-adult activity variation is also unrelated to polymorphisms among the lines for six esterase 6 allozymes and six restriction sites in a region encompassing the esterase 6 coding DNA and the first kbp of 5' flanking DNA. However, two insertions, of 8.0 and 6.8 kbp, located about 1.4 kbp 5' of the esterase 6 coding region are associated with low activity in larvae and, to a lesser extent, in pupae, albeit not in adults. Restriction mapping reveals similarity between the 8.0 kbp insert and the 7.4 kbp retrotransposon 17.6. The differences in larval activity among lines are positively correlated with fitness as assessed from assays of pre-adult viability and development time but no significant associations between pupal esterase 6 activity and these measures are detected. Some effects of esterase 6 allozyme differences are also found for viability and development time but these effects could be explained by linkage disequilibrium between the 8.0 kbp insert and the EST6-9 allozyme.

Associations of esterase 6 allozyme and activity variation with reproductive fitness in Drosophila melanogaster

Previous studies have shown that the esterase 6 (EST6) enzyme of D. melanogaster is mainly produced in the sperm ejaculatory duct of the adult male and comparisons of wild-type males with laboratory null mutants have suggested that the enzyme plays a role in reproductive fitness. In this study we have compared 18 field-derived lines each isoallelic for Est6 for differences in five components of male reproductive fitness. No consistent fitness differences were found among lines differing in respect of the two major allozyme classes EST6-F and EST6-S, despite other evidence that these two classes are not selectively equivalent in the field. However, differences in reproductive fitness were found among lines differing in the minor mobility variants that segregate within EST6-F and EST6-S. A failure to distinguish among these minor forms may explain the discrepancies in previous studies on the effects of the major EST6 allozymes on reproductive fitness. The most significant associations we have found between EST6 and reproductive fitness were due to variation in EST6 activity levels. Male EST6 activity levels were found to be positively correlated with their time to first mating, negatively correlated with the numbers of eggs laid and progeny produced by their mates, and negatively correlated with the frequency with which their mates remate. We conclude that some EST6 variants differ in components of male reproductive fitness operative in laboratory cultures. However, the evidence for fitness differences is stronger for variants affecting the amount, rather than the structure of the enzyme, and the direction of the differences varies between some of the fitness components tested.

Regulatory aspects of esterase 6 activity variation in sibling Drosophila species

Esterase 6 in Drosophila melanogaster, Drosophila simulans and Drosophila mauritiana is produced in several life stages and diverse tissues, but the major pulse of expression is in the sperm ejaculatory duct of adult males. Comparison of EST6 activity levels among several lines of D. melanogaster, D. simulans and D. mauritiana reveals two major quantitative differences among the species. First, newly eclosed females of both D. simulans and D. mauritiana show significantly higher EST6 activity than those of D. melanogaster. Secondly, 5-day-old adult D. simulans have significantly higher activities than D. mauritiana in both sexes and significantly higher activity than D. melanogaster in males. The genetic bases of the differences between D. melanogaster and the other species are investigated through germ line transfer of the D. simulans and D. mauritiana Est-6 genes plus 1.2 kb of 5' and 0.2 kb of their 3' flanking sequences into D. melanogaster. The newly eclosed female activities of the transformants resemble those of the two donor species, suggesting that the interspecific differences in this aspect of expression are due to cis-inherited factors contained within the transferred DNA. In contrast, the 5-day adult activity of the D. simulans transgene resembles the recipient species, D. melanogaster, suggesting that the difference between D. simulans and D. melanogaster in this aspect of expression is due to trans-acting factors. We also find that third instar larval activities of the D. simulans transgene and 5-day male activities of the D. mauritiana transgene are lower than those of either parental species, suggesting that not all the promoter elements relevant to these aspects of expression are included in the transferred DNA.

Evolutionary genetics of Drosophila esterases

Over 30 carboxylester hydrolases have been identified in D. melanogaster. Most are classified as acetyl, carboxyl or cholinesterases. Sequence similarities among most of the carboxyl and all the cholinesterases so far characterised from D. melanogaster and other eukaryotes justify recognition of a carboxyl/cholinesterase multigene family. This family shows minimal sequence similarities with other esterases but crystallographic data for a few non-drosophilid enzymes show that the family shares a distinctive overall structure with some other carboxyl and aryl esterases, so they are all put in one superfamily of/beta hydrolases. Fifteen esterase genes have been mapped in D. melanogaster and twelve are clustered at two chromosomal sites. The constitution of each cluster varies across Drosophila species but two carboxyl esterases in one cluster are sufficiently conserved that their homologues can be identified among enzymes conferring insecticide resistance in other Diptera. Sequence differences between two other esterases, the EST6 carboxyl esterase and acetylcholinesterase, have been interpreted against the consensus super-secondary structure for the carboxyl/cholinesterase multigene family; their sequence differences are widely dispersed across the structure and include substantial divergence in substrate binding sites and the active site gorge. This also applies when EST6 is compared across species where differences in its expression indicate a difference in function. However, comparisons within and among species where EST6 expression is conserved show that many aspects of the predicted super-secondary structure are tightly conserved. Two notable exceptions are a pair of polymorphisms in the substrate binding site of the enzyme in D. melanogaster. These polymorphisms are associated with differences in substrate interactions in vitro and demographic data indicate that the alternative forms are not selectively equivalent in vivo.

The number and distribution of esterase 6 alleles in populations of Drosophila melanogaster

High resolution electrophoretic analyses of the polymorphic esterase 6 enzyme have been carried out on 133 isoallelic lines from three Australian populations of Drosophila melanogaster spanning 25 degrees of latitude. These and previous data for 157 lines from another Australian population at an intermediate latitude reveal a total of 14 polymorphic esterase 6 allozymes, falling into five major mobility classes. Two classes, EST6-F and EST6-S, contain eleven of the allozymes but one allozyme, EST6-8 within the EST6-S class, is several times more common than any other. Variation in the frequency of this single allozyme can explain most of the latitudinal clines previously reported for the major EST6-F and EST6-S classes. Thermostability analyses of 52 of the Australian lines and 13 American lines reveal at least seven more EST6 variants within five of the allozymes, bringing the total number of variants to at least 21. Of the six allozymes for which more than one line was subjected to thermostability analyses, only EST6-8 could not be partitioned into additional variants. This corroborates a previous finding that two different isolates of the Est6-8 allele have identical DNA sequences and suggests that this allele, although now the most common, has nevertheless arisen relatively recently.