Increased Female Mortality As a Barrier To

JUNE 1987 Hygntolzmton Bennren tr.l :{gpps scvtELLARrs Corupr.px INCREASEDFEMALE MORTALITY AS A BARRIER TO HYBRIDIZATION BETWEEN MEMBERS OF THE AEDES SCUTELLARIS COMPLEX OF MOSQUTTOES R. E. DUHRKOPF Department of Bialagy, Ba.ylor Uniuersity, Waro, TX 76798 ABSTRACT. Interspecific crosses between the mosquitoeg Aedes polyncsiensis and Aedes mdnyensis have shown a unidirectiona_l pattern of compatibility. Aedes plynesicnsis females inseminated by Ae. nwlayercb males fail to produce viable offspring while the reciprocal cross is viable. In both crosses, rates of insemination are comparable to control rates. The Ae. polynesbnsis females fail to lay eggs.One apparent reason for thig is that the Ae. plynesiensis females have a high rate of mortality aftcr inseminatioiAy Le. malayensis males. Such mortality is an effective barrier to hybridization in that-croes, and is a new cliss of isolating mechanism. INTRODUCTION unidirectional pattern of compatibility (Yen and Barr 1973, Wright and Barr 1980, Wright anc The Aedes scutellaris complex of mosquitoes Wang 1980,Trpis et al. 1981b). is comprised ofabout 30 speciesdistributed from During attempts at these crosses it was ob- Andaman the Islands in the west to the Mar- served that the Aedespolynesi.ensis females in- quesas and Tuamoto Archipelago in the east, seminated by Ae. m.ahyenslsmales apparently and reaching as far north as Okinawa. Several had a higher rate of mortality than colony fe- members have been implicated as important males. This was true in a variety of attempts vectors of filariasis in the South Paciftc. Aedes involving several different laboratory strains. polyncsiensis Marks has been shown to be highly Even in attempted crossesof very large numbers susceptible to both Brueia Whnnqi and,Bngia (>500 females) mortality was so great that few, malayi (Duhrkopf and Trpis 1980).It is distrib- if any, eggswere laid. Such mortality could prove uted throughout the eastern South Paoific, to be a substantial isolating mechanism, pre- reaching as west far as the Ellice Islands and as venting interspecific hybridization in one direc- far east as the Marquesas, Tuamotos and Pit- tion through the post-fertilization death of the cairn Island (Macdonald 1976).Aed.es malayen- female prior to oviposition. This study is an srs Colless is refractory to filarial infection investigation of the extent of that post-fertiliz- (Macdonald 1976 and Trpis et al. 1981). It is ation mortality. distributed throughout the western South Pa- cific and Southeast Asia. The limits of its dis- MATERIALS AND METIIODS tribution are the Malaysian Peninsula on the east, the Andaman Islands on the west, and All mosquitoeswere from coloniesmaintained Vietnam and Thailand on the north. in the Laboratories of Medical Entomology at The factors involved in frlarial susceptibility The Johns Hopkins University School of Hy- have been shown to follow a nonMendelian pat- giene and Public Health. Aedes malayensis tern of inheritance (Trpis et al. 1981a). It is BANG strain was originally obtained from the possiblethat a rickettsial symbiont is associated SEATO Medical ResearchLaboratory in Bang- with filarial susceptibility (Duhrkopf and Trpis kok in 1969. Aedes polynesiensis APIA strain 1981).During the analysis ofthe genetic system was collectedby Barry Engber in Apia, Western involved in susceptibility, several crosseswere Samoain t977. All mosquitoeswere reared in a attempted between members of the complex. controlled environment of 26'C and 80% RH on Most of these crossesshowed a unidirectional a 16:8 light:dark regimen. pattern of compatibility. Of importance to this Larvae were reared in 29 x 18 cm rectangular paper is the seriesof reciprocal crossesbetween pans at a density of 100 per liter. Larvae were Aedes polyncsiensrs and.Ae. malayensis. When fed on diluted liver powder suspension. Upon Aedes rnal,ayensisfemales are inseminated by pupation, the pupae were removed and sexed. Ae. polynesicnsdsmales, viable hybrids result. The sexes were separated and held for emer- When Aedes polyrwsiznsis females are insemi- gence.Newly emerged adults were temporarily nated by Ae. malayensrrsmales, no viable off- held in cylindrical paper containers, 18 cm high spring are produced. Similar patterns have pre- x 18 cm diameter. All crosseswere set up with viously been reported in the Aedes scutellaris adults which were no more than 12 hours old. complex (Tesfa-Yohannesand Rozeboom1974, The initial experiment was to compare the Macdonald 1976),and inthe Culcx pipicns com- rates of insemination in control versus experi- plex (Laven 1951). In both cases,a rickettsial mental populations. Four groups of 20 cages symbiont has been reported as the causeofthe were set up. They were small, cylindrical paper JounNel oF THE AupRrclN Moseurro CoNrnol AssocnnoN VoL. B, No.2 cages,9 cm high x g cm in diameter. A vial of Table1. Percentagesofinsemination in controland water was inserted through a hole in the bottom, expedmentatfumal* piece and a of cotton soaked with l0% sucrose Aedespolywsi.ensis Aedesm.alnyensis was kept at the top. Five males and five females were placed in each cage.In one set of 20 cages, Experi- Experi- all males and femaleswere Ae. polynesi.ensis.In Day Control mental Control mental the secondset of 20 cages,all males and females 1 0000 wereAe. malayensis.In the third set of 20 cages, 2 0000 the males wereAe. polynesi.ensisand the females 3 1001010 were Ae. m,alo.yensis,and in the fourth set of 4 40 20 50 40 cages,the males were Ae. malayenslsand the o 80 70 90 90 6 100 100 100 100 females wete Ae. polyncsiensis.Every day, 10 n 100 100 100 100 females were randomly removed from various cagesand dissectedto inspect the spermathecae for the presenceof sperm. experimentalsand 78 andTt% in the controls). A similar procedure was used to investigate No such difference is seen in the data for the the rates of survival in the different populations. Aedes mal,ayensrsfemales (90 and 88% in the Eight groups of 20 small cageswere set up as experiments and 92 and96% in the controls). above. In the first two groups, all males ani In Figs. 1 and 2, the survival curyesare shown females werc Aedespolynesiensis. In the second. over the 20 day period. Fig. 1 presents the sur- two groups,all males and females wete Ae. tna- vival curves for the four populations of Aedes layensis. In the third two groups the males were polynesicnsis females, and Fig. 2 presents the Ae. polyncsiensrsand the females were Ae. ma- survival curves for the four populations of Ae. layensis,and in the final two groups, the males nwla.yensisfemales. The curves demonstrate the wete Ae. malayensisand the females were Ae. differences between the populations which were polynesiensis.For eachday over a 20 day period, presented in Table 2. It can be seen that the the number of surviving females in each cage curves for the two Ae. polyncsiazsrs experimen- was recorded.In this way, there were two repli- tal lines are different from those of the two cates for each group-Ae. polyrwsiensiscontrol control lines, while all four curves for the Ae. females (inseminated by Ae. polyrwsicnsis m,alayensisfemdes are similar. males),Ae. malayenslscontrol females (insemi- The Log-rank Survival Analysis results in a nated by Ae. malayensismales), Ae. malnryensis statistic which is distributed as a Chi-square. experimental females (inseminated by Ae. poly- Differences between the Aedes malayensis nesiensismales), and Ae. polynesicnsisexperi- groups were not significant (x2 : 3.93, 3 d.f.). mental females (inseminated by Ae. malayensis There were significant differences between the males).Analysis of the data was done using Log- Aed.espolynzslensis groups (x2 : 47.L6,3 d.f., P rank Survival Analysis (Anderson et al. 1980). < 0.001). Further comparisons of the Aedes polyrrcsiensisgroups showed no significant dif- (12 = RESULTS ference between the two control groups 0.78, 1 d.f.) and no significant difference be- The results of the insemination tests are tween the two experimentalgroups (x2 :3.I7, shown in Table 1. The purpose of the insemi. 1 d.f.). Thus, the experimental groups had a nation test was to determine whether femalesin significantly reducedsurvival when comparedto interspecific crosseswere being successfullyfer- the control groups. tilized. The results of the insemination tests show that, by the sixth day, all of the females DISCUSSION had sperm in their spermathecae.Thus, the Aedesrnalayensrs males were successfullyinsem- Aedes polyncsierxis has been the subject of inating the Ae. polynesiensisfemales, and Ae. intense study for the past 30 years becauseof polynesiensismales were successfullyinseminat- its susceptibility to filarial parasites. As previ- ing Ae. rnalayensisfemales. ously mentioned, crossesbetween Ae. polync- The results of the mortality experiments are sicnsh and Ae. mala.yensis have demonstrated shown in Table 2 and Figs. 1 and 2. In Table 2, an unidirectional pattern of compatibility the daily survival rates are presented for the (Tesfa-Yohannesand Rozeboom1974, Macdon- eight groups. From day 8 through day 20, the ald 1976, Trpis et al. 1981a). That pattern is number of females in the Aedes polyncsiensis believedto be due to the presenceofa rickettsial experimental populations was less than the symbiont (Wright and Wang 1980, Wright and number in the control populations. The finai Barr 1980, Trpis et al. 1981b). Unidirectional results were many fewer females in the experi- pattern of compatibility in membersof the Aedcs mentals than the controls (46 and 36%

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