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Los patrones de aparicin y polimorfismo masculino en la avispa no-polinizadora de los higos Aepocerus sp. (Torymidae)
Emergence patterns and male polymorphism in the nonpollinating fig wasp Aepocerus sp. (Torymidae)
This study was designed to investigate a perceived male dimorphism in the nonpollinating fig wasp Aepocerus sp. (Torymidae), as well as to examine emergence schedules of males and females as well as of different male morphs. I collected 100 Ficus pertusa figs and preserved the wasps that emerged from each fig every day for five days. I counted the number of male and female Aepocerus sp. that emerged from each fig each day, and I measured body size and degree of wing patchiness in males to assess whether the species exhibits male dimorphism. I found that males tend to emerge earlier (avg = 3.02 1.14 days, N = 528) than females (avg = 3.44 1.18 days, N = 340)(t = -5.601, p < 0.001). Male body sizes ranged from 1.1 to 2.5 mm with an average of 1.764 mm 0.20 (N = 528) and followed a roughly normal distribution. Across all males, wing patchiness was positively correlated with body size (R2 = 0.541, p < 0.001, N = 528) and body size was negatively correlated with day of emergence (R2 = 0.066, p < 0.001, N = 528). I observed two male morphs, distinguished most clearly by the appearance of their abdomens (rather than by body size or wing patchiness, as previously believed). Males with opaque abdomens tended to be smaller (avg = 1.57 0.02 mm, N = 78) than males with translucent abdomens (avg = 1.80 0.05 mm, N = 450)(t = 8.847, p < 0.001) and also tended to emerge later (avg = 3.65 1.32 days, N = 78) than males with translucent abdomens (avg = 2.91 1.02 days, N = 450)(t = -5.834, p < 0.001). My results highlight the need for further study of Aepocerus sp. in order to understand developmental mechanisms for male morphs.
Este estudio fue diseado para investigar un notorio dimorfismo masculino en la avispa no-polinizadora de los higos Aepocerus sp. (Torymidae), as como examinar los horarios de aparicin de los machos y las hembras, al igual que de los diferentes morfos masculinos.
Text in English.
Sexual dimorphism (Animals)
Tropical Ecology 2008
Nonpollinating fig wasp--Costa Rica
Ecologa Tropical 2008
t Monteverde Institute : Tropical Ecology
1Emergence Patterns and Male Polymorphism in the Nonpollinating Fig Wasp Aepocerus sp. (Torymidae) Katelyn Burgess Department of Biology, College of William & Mary ABSTRACT This study was designed to investigate a perceived male dimorphism in the nonpollinating fig wasp Aepocerus sp. (Torymidae), as well as to examine emergence schedu les of males and females as well as of different ma le morphs. I collected 100 Ficus pertusa figs and preserved the wasps that emerged from eac h fig every day for five days. I counted the number of male and female Aepocerus sp. that emerged from each fig each day, and I mea sured body size and degree of wing patchiness in males to asse ss whether the species exhibits male dimorphism. I found that males tend to emerge earlier (avg = 3.02 1.14 day s, N = 528) than females (avg = 3.44 1.18 days, N = 340)(t = -5.601, p < 0.001). Male body sizes ranged from 1. 1 to 2.5 mm with an average of 1.764 mm 0.20 (N = 528) and followed a roughly normal distribution. Across all males, wing patchiness was positively correlated w ith body size (R2 = 0.541, p < 0.001, N = 528) and body size was neg atively correlated with day of emergence (R2 = 0.066, p < 0.001, N = 528). I observed two male morphs, disti nguished most clearly by the appearance of their ab domens (rather than by body size or wing patchiness, as pr eviously believed). Males with opaque abdomens tend ed to be smaller (avg = 1.57 0.02 mm, N = 78) than males w ith translucent abdomens (avg = 1.80 0.05 mm, N = 450)(t = 8.847, p < 0.001) and also tended to emerge later ( avg = 3.65 1.32 days, N = 78) than males with tra nslucent abdomens (avg = 2.91 1.02 days, N = 450)(t = -5.8 34, p < 0.001). My results highlight the need for f urther study of Aepocerus sp. in order to understand developmental mechanism s for male morphs. RESUMEN Este estudio fue diseado para investigar un notori o dimorfismo en los machos de la avispa no-poliniza dora de higos Aepocerus sp. (Torymidae), as como examinar los tiempos de e closin de machos y hembras y de las diferentes formas de los machos. Colect 100 frutos de Ficus pertusa y se conservaron las avispas que eclosionaron de c ada higo cada da durante cinco das. Cont el nmero de hembras y machos de Aepocerus sp. que eclosionan de cada higo diariamente, y med el tamao corporal y el pa tron de manchas en las alas para calcular el dimorf ismo exhibido por los machos. Encontr que los machos tienden a eclosionar ms temprano (prom = 3.02 1.14 dias, N = 528) que las hembras (prom = 3.44 1.18 dias, N = 340)( t = -5.601, p < 0.001). El tamao corporal de los machos varia de 1.1 a 2.5 mm con un promedio de 1.764 mm 0.20 (N = 528) y sigue una distribucin normal. Entre t odos los machos, el patrn de manchas en las alas est corre lacionado positivamente con el tamao corporal (R2 = 0.541, p < 0.001, N = 528) y el tamao corporal esta negat ivamene correlacionado con el da de eclosin (R2 = 0.066, p < 0.001, N = 528). Observ dos morfos en machos, distinguidos claramente por la apariencia del abdo men (ms que por el tamao corporal o el patrn de manchas e n las alas, como crea previamene). Machos con abd omens pacos tienden a ser ms pequeos (prom = 1.57 0. 02 mm, N = 78) que los que tienen abdomens translc idos (prom = 1.80 0.05 mm, N = 450)(t = 8.847, p < 0.0 01) y tambien tienden a eclosionar ms tarde (prom = 3.65 1.32 dias, N = 78) que los machos con abdomens tran slcidos (prom = 2.91 1.02 dias, N = 450)(t = -5. 834, p < 0.001). Mis resultados sugieren un futuro estud io en Aepocerus sp. en orden para entender mecanismos de desarrollo en los morfos de los machos. INTRODUCTION Figs ( Ficus Family Moraceae) are some of the most abundant tr opical trees in the world. Globally, there are approximately 900 described spe cies, with at least 65 in Costa Rica alone
2 (Janzen 1979). They are characterized in part by t heir round, hollow inflorescences ( syconia ) which are lined with tiny florets containing a sing le ovary. It is inside these syconia that one of the most complex and fascinating mutualisms known o ccurs. Each species of fig is pollinated by a unique species of wasp (Family Agaonidae). When a female agaonid wasp arrives at a receptive syconium, she enters and, once inside, sh e crawls along the florets, laying eggs as well as spreading pollen carried from another fig. She dies once oviposition is complete, but her larvae develop, feeding off the ovary tissue. Once mature, the wingless agaonid males emerge from the ovaries first, remain in the hollow space where they seek females with whom they copulate. Males then cut an exit hole through the fig tissue and die soon thereafter. However, females, laden with pollen, exit through the hole t o seek another tree with receptive figs, where the cycle begins anew (Janzen 1983). Besides these pollinators, figs are also host to a number of nonpollinator wasp species. These nonpollinators are believed to be somewhat le ss host-specific, and there may be many different nonpollinator species associated with a s ingle species of fig. Among the nonpollinators are those of the family Torymidae. Rather than ovip ositing from within the syconium, torymid females use their ovipositors to puncture the sycon ium wall from the outside to reach unoccupied ovaries in which to deposit their eggs. Upon matura tion, torymids exit the syconium via the hole chewed out by male agaonids. Thus, torymids are hig hly dependent upon agaonids, both for their escape route and because the fig tree will likely a bort any syconia which are not pollinated (Bronstein 1991; West et al. 1996). Some torymids differ greatly from agaonids in term s of mating. Aepocerus sp. is one such species. These wasps are specific to Ficus pertusa a common fig tree in the Monteverde area. While agaonids copulate prior to the females exit from the syconium, Aepocerus sp. males and females emerge from the fruit first, then copulate outside. Additionally, this species exhibits extreme male polymorphism and associated alternativ e mating strategies which, despite their uniqueness and curiosity, have been poorly studied. Some males are larger with brownish patches on their forewings while others are smaller and have clear forewings. Larger males are aggressive and wait for emerging females by the syc onium exit hole, attacking competitors (Bronstein 1991), and smaller males wait at a dista nce from the exit hole and sneak copulations (K. Masters, personal communication). The degree to which these represent distinct morphs versus a continuum has not previously been studied. Assuming that there are in fact two distinct Aepocerus sp. male morphs with associated mating strategies, it is of interest to consider ho w such a situation arises. How these two behaviors can occur in one species has been debated as it would seem that one would be more successful and force the other out of existence (Ca de 1980). However, it appears that they may be examples of what are referred to as Evolutionari ly Stable Strategies (ESS). An ESS is a strategy which, when a certain frequency of the pop ulation adopts it, is unbeatable reproductively compared to a given set of alternati ves (Cade 1980). Evolution generally favors males who will act aggressively for immediate repro ductive payoff, meaning that associated traits like developed secondary sex characteristics will be selected for as well. This explains the existence of the larger, patchy-winged Aepocerus sp. males who exhibit fighting behavior. These males should have more copulations in a given time period relative to the smaller males, however they also experience the energetic costs of larger body size and patch development, as well as decreased lifespan due to fighting. On the other hand, smaller males should be competitive because they have lower energetic costs and increased survivorship. While they may not copulate as often as the larger males in a give n period of time, they should come out even
3 over their lifespans. The relative strategy freque ncies should theoretically be kept at equilibrium because the benefits of a strategy increase if its frequency decreases. Hence, they persist simultaneously because they balance one another. It is also of interest to consider how the male mo rphs develop, i.e. whether it is under genetic or environmental control, or some combinati on of the two. A number of studies have focused on organisms exhibiting similar male polymo rphisms to address this very question. For example, Emlen (1997) studied the beetle Onthophagus acuminatus which has male dimorphism as well as associated aggressive and sneaker mati ng strategies. His study concluded that higher diet quality resulted in the larger, more ag gressive male phenotype, regardless of parent phenotypes, suggesting a strong environmental contr ol. Additionally, Kurziel & Knowles (2002) investigated the amphipod Jassa marmorata which shows a similar trend in males, and concluded that a higher quality diet as well as a l onger development time led to the more aggressive male. Given these findings, I hypothesi zed that Aepocerus sp. male polymorphism is strongly influenced by environmental factors during development, including the time spent in development This study was designed to investigate the relation ship between the time of emergence from the syconium (i.e. development time) and the male morphs, as well as the correlation between male size and wing patchiness ( i.e. development time and degree of polymorphism). I predicted that there would be a st rong correlation between male body size and wing patchiness, and that larger, more aggressive m ales would emerge later than the smaller sneaker males. MATERIALS & METHODS I used a F. pertusa individual in the backyard of the Cspedes-Marin fa mily in Caitas, Costa Rica, near the entrance to la Finca Los Cruz. I col lected 100 figs with newly formed exit holes over the course of two days. Each fig was placed i n a separate jar. Once a day for five days, I removed the wasps that had emerged from each fig an d preserved them in ethanol in Petri dishes labeled by fig and day. I counted the number of mal e and female Aepocerus sp. individuals that emerged from each fig on each day. Day 1 represen ts the first day on which male Aepocerus sp. males emerged from figs. I also measured the m ales from head to abdomen, using a dissecting microscope fitted with an ocular microme ter. Finally, I scored the degree of patchiness of each males forewings. I found that m ales could have patches of variable size on the tips of their forewings, near their wing joints in both places, or in neither. Based on that, I made a scale from one to six in order to score each individual. The scale was as follows: 1 no patches; 2 very faint patches near wing joints; 3 small patches near wing joints; 4 distinct patches near wing joints; 5 small patches on wing tips and near wing joints; 6 strong patches on wing tips and near wing joints (Figure 1a-f, res pectively). I tested for a correlation between male body size and wing patchiness using a regression analysis. I also tested for a correlation between male body size and day of emergence using a regression analysis. I used t-tests to analyze: (1 ) the difference between average male and female day of emergence; (2) the difference between the av erage body sizes of the two male morphs; and (3) the difference between average days of emer gence for the two male morphs. RESULTS I collected 340 Aepocerus sp. females and 528 Aepocerus sp. males. Males tended to emerge earlier (avg = 3.02 1.14 days, N = 528) than fema les (avg = 3.44 1.18 days, N = 340)
4 (t = -5.601, p < 0.001)(Figure 2). Male body sizes ranged from 1.1 to 2.5 mm with an a verage of 1.764 mm 0.20 (N = 528) and followed a roughly no rmal distribution (Figure 3). I observed two male morphs: one with a fatter, opa que black abdomen and clear forewings (opaque morph, Figure 1a), and one with a narrower, translucent abdomen and variable wing patchiness (translucent morph, Figu re 1b-f ) Opaque morph males were generally smaller (avg = 1.57 0.02 mm, N = 78) th an translucent morph males (avg = 1.80 0.05 mm, N = 450)(t = 8.847, p < 0.001). Translucen t morph males generally emerged earlier (avg = 2.91 1.02 days, N = 450) than opaque morph males (avg = 3.65 1.32 days, N = 78) (t = -5.834, p < 0.001)(Figure 4). Across all males, wing patchiness was positively c orrelated with body size (R2 = 0.541, p < 0.001, N = 528)(Figure 5) and body size was neg atively correlated with day of emergence (R2 = 0.066, p < 0.001, N = 528)(Figure 6). DISCUSSION My results show that males generally emerge from fi gs earlier than females, which fits my original predictions. Aepocerus sp. males wait for females at exit holes as they e merge so that they can copulate with them, so it is logical that males would emerge first. Further, my results show that females emerged only a little less than h alf a day after males. The amount of time that a male has to wait outside a fig for females to eme rge presumably has implications for that males fitness, since he is susceptible to predator s and environmental threats in the meantime (Bronstein 1988). This short lag time may have evol ved in order to give most males just enough time to leave the fig before females emerge, minimi zing time spent exposed on the fig surface. It would be interesting to assess the rate of male mor tality in Aepocerus sp. as they wait for females, relative to their own days of emergence. The male phenotype has proven to be more complex t han previously understood. Bronstein (1991) described a male dimorphism distin guished by body size and the presence or absence of wing patches. Specifically, she noted a small, clear-winged morph and a large, patch-winged morph. However, my results show that male body size is continuous and positively correlated with a continuum of wing patc hiness, suggesting that size or wing patchiness alone are not enough to distinguish betw een morphs. I did note a distinct male dimorphism, but one that differed somewhat from Bro nsteins. One morph that I saw (probably roughly the one Bronstein referred to as small, cl ear-winged) was generally smaller and always had clear forewings, but, notably, also had a fatte r, more opaque abdomen, which I found to be the most distinctive characteristic of the morph. T he other male morph that I observed had a narrower, more translucent abdomen as well as forew ings exhibiting a continuum from clear to very patchy. Henceforth, the morphs will be referr ed to as opaque and translucent, respectively, to reflect the most clearly distingui shing characteristic between them. Within the translucent morph, there is considerabl e polymorphism. Wing patchiness increases along with body size, so the smallest tra nslucent morph males tend to have very small, faint patches or none at all, and the biggest trans lucent morph males tend to have large, distinct patches. It seems then that I have observed a male dimorphism in Aepocerus sp., with the translucent morph exhibiting further polymorphism i n terms of wing patchiness. It is of interest to consider how such male phenot ypic variability arises developmentally. Bronstein (1991) noted that large, patch-winged m ales (roughly equivalent to my translucent morph) employ an aggressive mating tactic, defendin g the exit hole against competitors. Small,
5 clear-winged males (roughly my opaque morph) were observed to wait farther off to sneak copulations. While I did not include a behavioral s tudy in my work, I noted the same trends in casual observations. These factors combinedthat t he translucent morph males are generally larger, patchier, and more aggressivewould appear to suggest that they might emerge later than the opaque morph males, since it seems as though th ey would require more time to develop. On the contrary, my results indicated that translucent morph males generally emerged almost a day earlier than opaque morph males, suggesting that de velopment time alone is not responsible for morph determination. Of course, it is possible that the translucent mor ph males were not laid at the same time as the opaque morph males, or even by the same fema le. Or, maybe the translucent morph males are larger, patchier, and more aggressive because t hey had greater access to resources in development. The effects of differential nutrition in development of fig wasps has not been studied, but in studies on amphipods and beetles th at show similar male dimorphism and associated alternative mating strategies, better nu trition led to larger, more aggressive male morphs (Emlen 1997; Kurziel and Knowles 2002). If resources are the deciding factor here, those r esources could come from two possible sources. First, it may be that nutrient availabili ty is not uniform throughout a fig, so that two wasps in a single fig receive unequal resources and hence develop at different rates and to different degrees. If this were true, however, one could expect to find an equal degree of polymorphism in female Aepocerus sp. and in other species of fig wasps, unless Aepocerus sp. male development just happens to be more strongly a ffected by resource availability. Besides the resources from figs, it could also be that male s receive different amounts of nutrients from their eggs. Perhaps female Aepocerus sp. invest more in some eggs than others, leading those larvae developing from eggs with higher nutrients t o grow bigger and more quickly than males developing from lower quality eggs. Expanding on this idea, if female Aepocerus sp. have lowerand higher-quality eggs, it would make sense that they might oviposit their better eggs first and their lower-quality ones later on. If so it could help explain why the opaque morph males emerged later than the translucent morph male s. The results of my study raise a number of question s about Aepocerus sp., which only serve to highlight how little is known about this f ascinating species. Further research is needed on the developmental mechanism or mechanisms by whi ch the male dimorphism and translucent male polymorphism arise. It would be of interest i n the future to conduct studies assessing the degree of genetic relatedness between male morphs, or to manipulate nutrient availability for Aepocerus sp. male larvae to test for developmental effects. Both studies would help to shed some light on the mysteries of male morph determina tion in this species. ACKNOWLEDGEMENTS My sincere thanks to Karen Masters for introducing me to this amazing species, for helping me to desig n my study, and for lending her support and enthusiasm along th e way. Thanks to the Cspedes-Marin family for use of their fig tree, and especially to Anglica and Alejandra for helping me to collect figs. Thanks to Taegan McMaho n and Pablo Allen for helping me re-learn statistics. Thanks to the Estacin Biolgica de Monteverde for providing me with the
6resources to conduct my project and a gorgeous loca tion in which to do it. And finally, a huge thank y ou to my parents for sending me off to the rainforest. It ha s been quite an adventure. LITERATURE CITED BRONSTEIN, J. L. 1988. Predators of fig wasps. Biot ropica. 20(3): 215-219. -----------. 1991. The nonpollinating wasp fauna of Ficus pertusa : exploitation of a mutualism? Oikos. 61: 175-186. CADE, W. 1980. Alternative male reproductive behavi ors. The Florida Entomologist. 63(1): 3045. EMLEN, D. J. 1997. Diet alters male horn allometry in the beetle Oncophagus acuminatus (Coleoptera: Scarabaeidae). Proceedings of the Roy al Society of London: Biological Sciences. 264(1381): 567-574. JANZEN, D. H. 1979. How to be a fig. Annual Review of Ecology and Systematics. 10: 13-51. ----------. 1983. Blastophaga and other Agaonidae. In D. H. Janzen (Ed.). Costa Rican Natural History, The University of Chicago Press, Chicago, Illinois, pp. 696-700. KURDZIEL, J. P. AND L. L. KNOWLES. 2002. The mechanics of morph deter mination in the amphipod Jassa : Implications for the evolution of alternative mal e phenotypes. Proceedings of the Royal Society of London: Biolog ical Sciences. 269(1502): 1749-1754. WEST, S. A., E. A. HERRE, D. M. WINDSOR AND P. R. S. GREEN. 1996. The ecology and evolution of New World non-pollinating fig wasp co mmunities. Journal of Biogeography. 23(4): 447-458. FIGURES
7 FIGURE 1. Representatives of the wing-patchiness sc ale in Aepocerus sp. males from 1 to 6 (a-f, respectively). (a) is an opaque m orph; (b-f) are translucent morphs. (Photos not of equal magnification.) 0 50 100 150 200 250 12345 Day of emergence# Individuals Males Females FIGURE 2. Number of male and female Aepocerus sp. that emerged from figs each day. Males (N = 528) generally emerged ea rlier than females (N = 430)(t = -5.601, p < 0.001). a b c d e f
8 0 50 100 150 200 250 1.1-1.31.4-1.61.7-1.92.0-2.22.3-2.5 Body size (mm)# Individuals FIGURE 3. Aepocerus sp. male body size followed a roughly normal distribution (N = 528). 0 20 40 60 80 100 120 140 160 180 12345 Day of emergence# Individuals Opaque Morphs Translucent Morphs FIGURE 4. Number of Aepocerus sp. opaque morph males (N = 78) and translucent morph males (N = 450) that emerged from figs each day. Translucent morphs generally emerged earlier than opaque morphs (t = -5.834, p < 0.001).
9 0 1 2 3 4 5 6 7 0.81.31.82.32.8 Body size (mm)Wing patchiness FIGURE 5. Relationship between male body size a nd wing patchiness in Aepocerus sp. Wing patchiness tended to increase along with male body size (R2 = 0.541, p < 0.001, N = 528). 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 0123456 Day of emergenceBody size (mm) FIGURE 6. Relationship between day of emergence from figs and body size in Aepocerus sp. males. Larger males tended to emerge earlier th an smaller males (R2 = 0.066, p < 0.001, N = 528).