Brittnacher

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Paul M. Sharp

Prout

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Competitive mating in
                    Drosophila melanogaster

Selective differences among male Drosophila melanogaster due to differences in ability to compete for mates may often have been under-estimated in the past because, under the test procedure used, females did not represent a limited resource. In the experiment reported here, no difference was detected between inbred and outbred males ‘competing’ to mate with an equal number of females. When the receptive female: male ratio was halved a large reduction in male mating ability due to inbreeding became apparent

Sharp, Paul M.

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     Edinburgh Research Explorer                                      Competitive mating in Drosophila melanogasterCitation for published version:Sharp, PM 1982, 'Competitive mating in Drosophila melanogaster' Genetics Research, vol 40, no. 2, pp.201-205. DOI: 10.1017/S0016672300019066Digital Object Identifier (DOI):10.1017/S0016672300019066Link:Link to publication record in Edinburgh Research ExplorerDocument Version:Publisher's PDF, also known as Version of recordPublished In:Genetics ResearchPublisher Rights Statement:RoMEO greenGeneral rightsCopyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s)and / or other copyright owners and it is a condition of accessing these publications that users recognise andabide by the legal requirements associated with these rights.Take down policyThe University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorercontent complies with UK legislation. If you believe that the public display of this file breaches copyright pleasecontact openaccess@ed.ac.uk providing details, and we will remove access to the work immediately andinvestigate your claim.Download date: 28. Apr. 2017Genet. Res., Camb. (1982), 40, pp. 201-205 2 0 1With 1 text-figurePrinted in Great BritainSHORT PAPERSCompetitive mating in Drosophila melanogasterBY PAUL M. SHARPInstitute of Animal Genetics, West Mains Road, Edinburgh EH9 3JN, Scotland(Received 18 March 1982)SUMMARYSelective differences among male Drosophila melanogaster due to differencesin ability to compete for mates may often have been under-estimated in the pastbecause, under the test procedure used, females did not represent a limitedresource. In the experiment reported here, no difference was detected betweeninbred andoutbred males 'competing' to mate with an equal number of females.When the receptive female:male ratio was halved a large reduction in malemating ability due to inbreeding became apparent.INTRODUCTIONA large body of work has been devoted to the estimation of the components of fitnessin Drosophila melanogaster. Sexual selection, arising from differential success among malescompeting for mates, is recognized as being of major importance in this species (Prout,1971; Bundgaard & Christiansen, 1972). Any fitness measurement is likely to bedependent upon the circumstances under which it is estimated, so that real differencesin fitness may be overlooked if the competitive conditions are not appropriate. I wouldsuggest that one technique that has often been used to assess male mating abilities (forexample recently by Brittnacher, 1981; Clark & Feldman, 1981), in which males'compete' to mate with an equal number of females, may be unsatisfactory for tworeasons.(1) In any continuous population the number of adult males seeking mates is likelyto greatly exceed the number of females receptive at that time. Males are always willingto mate, but the majority of females, once inseminated, resist re-mating for a period of4-6 days (Manning, 1962; Pyle & Gromko, 1978).(2) The competitive ability of the weaker group is likely to be overestimated. This arisesbecause copulation in D. melanogaster lasts for about 20 min (Fowler, 1973). If, as isusually observed, most of the stronger competitors are successful soon after the start ofthe test, then while they remain in copula the weaker group of males may court theresidual females without significant competition.In order to verify this second point 1 have tested inbred and outbred males, expectedas a consequence of their genotypes to differ in competitive ability, against a standardmutant strain, under two sets of experimental conditions expected to provide differentintensities of competition.202 P. M. SHARP2. MATERIALS AND METHODSAll flies were derived from the Prevosti population cage, which had been initiated froma large collection made in the Canary Islands by Professor A. Prevosti. A large outbred'scarlet' population was set up by inserting the st (scarlet-eyed) gene into the Prevostibackground, by a combination of balanced marker chromosome and back-crossingtechniques. Ten inbred lines were derived from the Prevosti population by full sib mating,and were tested after one generation (i.e. the males tested had an inbreeding coefficientF = 0-25). All flies used in tests were reared under conditions of minimal competition,and all anaesthesia was by CO2. All flies were 3-6 days old when tested, and all groupsof competing males were matched for age.The basic test regime involved 25 wild-type (outbred or inbred) males and 25 st malescompeting to mate with 50 st females. There were two types of tests:(1) '50 $$': 50 females were shaken into the test arena with the 50 males at thebeginning of the test.(2) '25 $$ + 25?9': 25 females were added at the start, and a further 25 females 2 hlater.Approximately 5 h after the start of the tests the females were placed in individualvials, so that the genotype of their mate could be deduced from their progeny.Under each set of conditions, two sets of 25 <$<$ from each of five of the inbred lineswere each tested twice (on consecutive days). Exactly equivalent experiments were alsoconducted with outbred 'lines', so that in all some 4000 matings were monitored.3. ANALYSIS OF RESULTSThe male mating ability or virility of a group of males, relative to a specified standard,may be defined as a male competitive index, CI S (Latter & Robertson, 1962). If Q andP represent the number of females inseminated by wild-type and st males respectively,thenCI c? = p- Also CI S = ~r^zwhere q is the proportion of females mated by wild-type males (q = Q/(Q + P)). Resultsof replicate estimates are combined using q, as this has preferred statistical properties.The standard error of this index was also derived by Latter & Robertson (althoughmisprinted there):where a2 is the variance of the repeated measurements of q, and n the number ofreplicates.The mean proportions of females inseminated by each of the tested lines are presentedin Table 1. These are averaged over lines and converted into competitive indices in Table2 and Fig. 1. It may be seen that when there was a 1:1 ratio of females to males at thestart of the test, the competitive index of neither the outbred nor the inbred males differssignificantly from 1-0 (no apparent selection). The estimates for the two types of malesdo not differ significantly from each other (t8 = 0-98, P >0-20). Under the revised set oftest conditions the outbred males fare much better than either the st stock or the inbredmales, the reduction due to inbreeding being highly significant (ts = 4-00, P < 0-005).Short paper 203Table 1. Mean proportion, q (and s.D.) of females inseminated by ivild-type males,under two different test conditions. Each mean is derived from four tests(i)50?9 (ii) 25 99 + 259?Outbred0-531 (0021)0-478 (0-027)0-445 (0058)0-435 (0-075)0-536(0051)Inbred0-503 (0054)0-524 (0053)0-489 (0-045)0-517 (0039)0-495 (0019)Outbred0-516 (0-023)0-576 (0099)0-572 (0072)0-588 (0-082)0-595 (0056)Inbred0-488 (0043)0-456 (0047)0-409 (0077)0-531 (0018)0-488(0-051)Table 2. Male competitive index, Cl<§, and standard error, for outbred and inbredmales under different test conditions50 99 25 99 + 25 990-942 (0079) Outbred 1320 (0075)1024 (0027) Inbred 0-901 (0073)1-41-2•o100-850 99 25 99 + 25 99Figure 1. Effect of male:female ratio on male competitive index, CIcJ. '50 99 ' : 1:1male:receptive female ratio; '2599 + 2599 ' : 2:1 male:receptive female ratio. O,Outbred males; A, inbred males.GRH 40204 P. M. SHARP4. DISCUSSIONMultiple-choice competitive mating experiments are commonly a part of investigationsof fitness in Drosophila. Care must be taken, however, in ensuring that the experimentalconditions actually do induce inter-male competition. The results presented here suggestthat when a group of males are supplied with an equivalent number of females little orno competition will occur. Under a set of conditions designed to intensify competitiona previously undetected effect of reduced male mating ability, due to inbreeding, becameapparent. Subsequent work has confirmed the estimate of CTcJ for outbred flies under therevised procedure, and has also shown that continued inbreeding further reduces thiscomponent of fitness (Sharp, unpublished).The importance of male mating ability as a component of fitness has previously beenstressed (e.g. Parsons, 1974). Under certain experimental conditions it appears to be themajor factor influencing gene frequency changes (Prout, 1971; Bundgaard & Christiansen,1972). Although selective differences have been detected by these and other authors(Latter & Robertson, 1962; Aslund & Rasmuson, 1976; Clark & Feldman, 1981;Brittnacher, 1981) using the 1:1 male: female ratio, these differences may well have beenunderestimated by this technique. As a consequence the role of male mating ability innatural selection in D. melanogaster may yet have been understated.An alternative revision of the testing procedure would simply be to use only one-halfthe number of females. However, since in this test scheme the unit of measurement isa single insemination, the ensuing reduction in the fineness of the scale of measurementwould be undesirable. I t is probable that under our revised test conditions most, if notall, of the first batch of females have been mated before the second batch are added, andit is proposed that competition for the second group of females is quite independent ofthe first half of the test. To reflect truly the extent of competition in the wild wouldprobably require the use of an even lower receptive female:male ratio, althoughecological evidence on this point is lacking. That selection for mates does occur in thewild among male D. pseudoobscura has been demonstrated (Anderson et al. 1979).I wish to thank Dr Linda Partridge for critical reading of the manuscript. This work wassupported by a Science Research Council Postgraduate Studentship, under the supervision ofProfessor Alan Robertson.REFERENCESANDERSON, W. W., LEVINE, L., OLVERA, 0., POWELL, J. R., DE LA ROSA, M. E., SALCEDA, V. M.,GASO, M. I. & GUZMAN, J. (1979). Evidence for selection by male mating success in naturalpopulations of Drosophila pseudoobscura. Proceedings of the National Academy of Sciences,U.S.A. 76, 1519-1523.ASLUND, S-E. & RASMUSON, M. (1976). Mating behaviour as a fitness component in maintainingallozyme polymorphisms in D. melanogaster. Hereditas 82, 175-178.BBITTNACHER, J. G. (1981). Genetic variation and genetic load due to the male reproductivecomponent of fitness in Drosophila. Genetics 97, 719-730.BUNDGAARD, J. & CHRISTIANSEN, F. B. (1972). Dynamics of polymorphisms. 1. Selectioncomponents in an experimental population of Drosophila melanogaster. Genetics 71, 439-460.CLARK, A. G. & FELDMAN, M. (1981). The estimation of epistasis in components of fitness inexperimental populations of Drosophila melanogaster. II. Assessment of meiotic drive, viability,fecundity and sexual selection. Heredity 46, 347-377.FOWLER, G. L. (1973). Some aspects of the reproductive biology of Drosphila: sperm transfer,sperm storage, and sperm utilization. Advances in Genetics 17, 293-360.Short paper 205LATTER, B. D. H. & ROBERTSON, A. (1962). The effects of inbreeding and artificial selection onreproductive fitness. Genetical Research 3, 110-139.MANNING, A. (1962). A sperm factor affecting the receptivity of Drosophila melanogaster females.Nature 194, 252-253.PARSONS, P. A. (1974). Male mating speed as a component of fitness in Drosophila. BehaviorGenetics 4, 395-404.PROUT, T. (1971). The relation between fitness components and population prediction inDrosophila. I. The estimation of fitness components. Genetics 68, 127-149.PYLE, D. W. & GROMKO, M. H. (1978). Repeated mating by female Drosophila melanogaster: theadaptive importance. Experientia 34, 449-450.

Competitive mating in Drosophila melanogaster

Competitive mating in Drosophila melanogaster

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