InSectes Sociaux, Paris Masson, Paris, 1987 1987, Volume 34, n ~ 2, pp. 75-86

THE REPRODUCTIVE DIVISION OF LABOUR IN THE QUEENLESS PONERINE ANT RHYTIDOPONERA SP. 12

C.P. PEETERS School of Zoology, University of New South Wales, P.O. Box 1, Kensington N.S.W., Australia 2033 Re~u le 15 octobre 1986 Accept6 le 9 mars 1987

SUMMARY

Mated workers have replaced the queen as the functional egg-layers in several species of ponerine ants. In such queenless species, the reproductive status of workers can only be determined through ovarian dissections. However, the presence of yolky oocytes is not necessarily an indication of active egg-laying. In Rhytidoponera sp. 12, a substantial proportion of the workers confined under, ground have many large yolky oocytes in their ovaries. Examination of various details of oogenesis (size and appearance of basal oocytes,...) revealed that oocytes do not mature in unmated workers, and that they accumulate in the ovaries. In the 21 mated workers found in a colony excavated in October 1985, there were fewer large yolky oocytes, and the dense accumulations of yellow bodies indicated that eggs were laid regularly. Thus reproductive differentiation among the workers is controlled by insemination. The lack of egg-laying activity by unmated workers was confirmed by monitoring brood production in the laboratory. Trophic eggs do not seem to exist. Various characteristics of worker reproduction in ponerine ants are discussed.

RESUME

La division des rSles reproducteurs chez Rhytidoponera esp~ce 12, une fourmi Pon6rine sans reine Des ouvri6res fdcond6es ont remplac6 la reine (darts sa fonction de pondeuse) chez plusieurs esp6ces de fourrnis pondrines. Darts ces esp~ces sans reine, l'6tat reproductif des ouvri6res ne peut 6tre d6termin6 qu'avec la dissection des ovaires. Cependant, la prdsence d'ovocytes en vitellogen~se n'indique pas n6cessairement qu'il y ait ponte d'o~ufs. Chez Rhytidoponera esp6ce 12, une grande proportion des ouvri6res actives ~t l'int4rieur du nid ont beaucoup d'ovocytes en vitellogen6se darts leurs ovaires. L'examen de certains d6tails de l'ovogen6se (talUe et apparence des ovocytes basaux..) a d6montr6 76 C.P. PEETERS que les ovocytes ne mflrissent pas chez les ouvri~res non f6cond4es, et qu'ils s'accumulent dans les ovalres, Chez les 21 ouvri~res f4cond4es trouv4es dans une colonie d4terr6e en Octobre 1985, il y avait moins de gros ovocytes en vitellogen~se, et les accumulations denses de corps jaunes indiquaient que les ceufs 4taient pondus r6guli~rement. Donc la s6paration des r61es parmi les ouvri~res est contr616e par la f6condation. L'absence de ponte par les ouvri6res non f4cond4es a 4t4 confirm4e par 1'4tude du d4veloppement du couvain darts des groupes de fourmis au laboratoire. Les 0eufs trophiques ne semblent pas exister. Certaines caract4ristiques de la reproduction ouvri~re chez les fourmis pon4rines sont discut4es.

INTRODUCTION

The queen caste does not exist in various species of ants belonging to the sub-family Ponerinae, and ilthe reproductive function is perforce carried out by the workers. In OphCtialmopone berthoudi, egg-laying is restricted to the mated workers, and these produce both workers and males (PEETF.RS and Cm~wv., 1985a). Thus insemination controls the reproductive division of labour. In contrast, PAMILO et al. (1985) reported that, in Rhytidoponera sp. 12, a majority of the workers (both mated and unmated) exhibit ovarian activity, and that mated workers have less active ovaries than unmated workers from deep regions of the nests. This then implies that unmated workers also lay eggs, although PAMILO et al. concluded that these unfertilized eggs may have atrophic function. CROZIER et al. (1984) used allozyme data to establish the presence of h number of egg-layers in colonies of Rhytido- ponera sp. 12. This paper aims to highlight the difficulties involved in the assessment of reproductive activity in ponerine worker societies, where the mated egg- layers (= gamergates, PEETERS and CREWE, 1984) are morphologically iden- tical to their infertile nestmates.

MATERIAL AND METHODS

The population of Rhytidoponera sp. 12 ANIC studied (from Fowler's Gap Research Station, western New South Wales) is the same as that described in CROZIER et al. (1984) and PAMILO et al. (1985). An exceptionally populous colony was excavated on 7 October 1985, and the inhabitants were collected in groups according to the areas of the nest in which they had been found (jig. 1). During theevening prior to the nest excavation, workers present outside the nest entrance were collected separately. In addition, 17 foragers active over the previous days had been colour-marked. Due to time limitations, only a sample of this colony was dissected (table I). Only 10 of the " above ground " workers were dissected, but additional workers active in an identical context (including foragers) were collected in April 1986 and dissected. While DIVISION OF LABOUR IN RHYTIDOPONERA 77

20 MOUND

ground layer

li<

oha er -20

im I~ chamber 2

-40 I I

shaft " chamber -60 11 i~, chamber 3

Fig. 1. -- Schematic cross-section of the nest excavated on 7 October 1985. Fig. 1. -- Coupe sch6matique au travers du hid d6terr4 le 70ctobre 1985.

all the workers from chamber 2 were dissected, the shaft chamber was not sampled. Two sampling procedures were used to estimate the total number of gamergates in the remaining groups: (1) random - workers from chamber I were mixed together in a container and half of them were taken and dissected. (2) subjective - workers from chamber 3 were selected for dissection on the basis of their behaviour (e.g. active in the open, or confined to dark areas). The ants were kept in a laboratory nest consisting of a sheet of red perspex lying on top of a depression in soil; they were fed on meal- worms and honey water. Notes were made on the pattern of oocyte development in each worker dissected, and the spermathecae were examined in part of the sample.

RESULTS

A large proportion (82 %) of ~he workers dissected (n = 296) were found to have medium--large yolky oocytes in their ovarioles. However, only 21 inseminated workers were found. During the course of the dissections, consistent differences in ovarian activity were perceived between mated and unmated workers. Eventually, a set of diagnostic characters allowed the recognition of the ovaries of mated workers without prior examination of their spermathecae. The principal criterion for recognizing mated workers was the presence of large, dense accumulations of dark-yellow inclusions at the base of the ovaries. In contrast, only tiny light-yellow inclusions were found in the ovaries of most of the unmated workers. These inclusions appear to be 78 C.P. PEETERS yellow bodies ("corpus lutea") which are produced when nurse cells (" trophocytes ") degenerate (BILLEN, 1985). This normally occurs whenever a basal oocyte passes into the oviduct, and thus the conspicuous aggregation of yellow bodies found at the base of almost every ovariole in mated workers is evidence of active egg-laying. Yellow bodies can also be produced during oocyte resorbtion (e.g. MINKENBEI~G and PETIT, 1985), and this would account for their presence in very small quantities in unmated workers. Rare ovipo- sition by unmated workers cannot be totally excluded, but was never observed in the laboratory. In addition to yellow bodies, there were distinct differences in the pattern of oocyte growth between mated and unmated workers (except in one individual). One class of workers had many large, opaque (i.e. with yolk deposits) oocytes in their ovaries; there were up to four of these oocytes in each of the six ovariotes (fig. 2). In each ovariole, the basal oocyte was markedly larger than the next yolky oocyte ; usually it was abnormally- shaped (round, not oval), and bigger than eggs collected from the nest. The yolky oocytes in each ovariole were often squeezed together, and as a result their shapes were distorted. Sometimes adjacent oocytes were fused together. Finally, the internal macrostructure of these oocytes (especially the basal ones) was abnormal; the yolk had a mottled appearance, and this became more pronounced after a few minutes in a Ringer's solution. Workers with these ovarian characteristics ,were invariably found to have empty spermathecae (n ---- 123). In contrast, there were much fewer yolky oocytes in the ovaries of mated workers, and as a result they appeared less " active ". There were seldom more than four mature basal oocytes per individual, and one or more ovarioles often contained no yolky oocytes at all (fig. 3). Mature basal oocytes had regular shapes and sizes (relative to eggs), and yolk consistency was normal. In the few ovarioles that contained two opaque oocytes, the second oocyte was markedly smaller than the basal one. They were not squeezed together, and often nurse cells could be seen in the space between successive oocytes. I conclude that the oocytes in unmated workers do not mature. They are never laid, and thus they accumulate in the ovaries giving an impression of high " activity". In mated workers however, mature basal oocytes are laid regularly, and there is a dynamic turn-over of material. Thus an ovariole without several large yolky oocytes is not necessarily less active. These results indicate that there is an association between insemination and repro- ductive activity. Many of the workers active in the nest entrances after nightfall also had ovaries with many yolky oocytes in them. All were unmated. However, foragers showed definite signs of oocyte resorbtion. In most cases, only the basal oocytes remained, and the subsequent ones had either disappeared or DIVISION OF LABOUR IN RHYTIDOPONERA 79

Fig. 2 --Cl0se,u p of a developed ovary from an unmated worker. Note that there are a few opaque oocytes squeezed together in each of the three ovarioles, and that the yolk in tttelbasal oocytes has a mottled appearance. (scale 1 mm = 3 cm).

Fig. 2. -- Gros plan d'un ovaire d6velopp6 chez une ouvri~re non f6cond6e. On remarquera qu'il y a plusieurs ovocytes en vitellogen6se entass6s dans chacun des trois ova- rioles et, que levitellus des ovocytes basaux n'est pas homog6ne. 80 C.P. PEETERS

Fig. 3. -- Ovaries of a mated worker. Note the regular ovoid shapes of the basal oocytes. Only one opaque oocyte is found in most of the ovarioles. The arrows indicate the location of the yellow bodies, which are invisible in black and white photographs. (scale 1 mm = 2,5 cm).

Fig. 3. -- Ovaires d'une ouvri6re f6cond6e. On remarquera que les ovocytes basaux ont une forme ovo~de tr6s r6guli6re et qu'i! n'y a qu'un seul ovocyte en vitellogen~se dans la plupart des ovarioles. Les fl~ches indiquent l'emplacement des corps jaunes invisibles sur une photo en noir et blanc. DIVISION OF LABOUR IN RHYTIDOPONERA 81

Table I. -- Brood composition, and reproductive status of a sample of workers from a colony excavated in October 1985. No Cocoons were found at this time of the year. Workers were collected in discrete groups as the excavation progressed (see fig. 1). The spermathecae of a proportion of the infertile workers were also examined in order to verify the association between insemination and egg-laying: all were empty. Tableau I. -- Composition du couvain et 6tat physiologique des ouvri~res provenant d'une colonie d6terr4e en Octobre 1985. II n'y a aucun cocon h cette p6riode de l'ann4e. Les ouvri~res ont 6t6 r~colt6es sdpar6ment selon les poches indiqu4es sur la figure 1. Les sperrnath6ques d'une proportion des ouvri6res non pondeuses ont 6t6 aussi examindes afin de vdrifier l'association entre fdcondation et activit6 de ponte: elles 4taient routes vides.

Area of nest No. of No. of No. of No. of No. of No. of infertile eggs larvae workers workers gamergates workers checked dissected (all mated) for sperm (and % of total) above ground -- -- 81 53 $ 0 28 (53 %) chamber I 343 174 109 53 * 3 28 (56 %) chamber 2 136 57 95 94 4 47 (52 %) chamber 3 98 26 159 96 14+ 54 (66 %) shaft chamber 129 53 99 0 -- -- unknown -- -- 24 0 -- -- Total 706 310 567 296 21 157 (57 %)

$ collected from another colony * random sample + in one of these, spermathecae could not be checked.

were smaller than usual. The ovaries of some foragers contained no opaque oocytes at all. Within the laboratory colony (chamber 3), unmated workers which had become active in the open also exhibited regressed ovaries. The numbers of gamergates and infertile workers dissected in the various parts of the nest are presented in table I. Gamergates were present in chambers 1, 2 and 3, but were concentrated in the latter. There was no obvious correlation between the amount of eggs and larvae and the number of gamergates found in each chamber. This confirms the notion that brood is moved around within the nest. Most of the inhabitants of chamber 3 were kept alive for a few months before they were dissected. Eggs were laid and were reared to cocoons (31 cocoons opened and sexed contained worker pupae exclusively), and adults (all workers). No males were produced in the laboratory, Egg-laying was monitored in this and in other groups of workers, and rates of egg production were estimated for single gamergates (table II). These Values are averages for several gamergates, and thus they conceal individual va- riability. In laboratory nests, gamergates were always confined to dark and less accessible areas, and they reacted timidly to any disturbance. 82 C.P. PEETERS

Table II. -- Rates of egg-laying in various groups of workers isolated in the laboratory. While oviposition was monitored in a group, workers were dissected in order to determine their reproductive status. A new observation period was usually started whenever gamergates were dissected. Tableau II. -- Cadence de ponte dans quelques groupes d'ouvri6res isol6es au laboratoire. Pendant que la ponte 6tait mesur~e pour un groupe, les ouvri6res 6taient diss~qu6es afin de d6terminer leur 6tat reproductif. Uric nouvelle p~riode d'observation 6tait d'habitude commenc6e quand des gamergates 6taient diss6qu~es.

Colony Time No. of No. of Estimates of group in eggs gamergates egg-laying rate isolation laid present chamber 1 12 days 34 3 0,94 eggs/gam./day 17 days 28 3 0,55 eggs/gam./day chamber 2 8 days 43 4 1,3 eggs/gam./day 3 days 17 2 2,8 eggs/gam./day chamber 3 16 days 125 14 0,56 eggs/gam./day 26 days 290 13 0,86 eggs/gam./day 41 days 118 8 0,36 eggs/gam./day Average 11,05 eggs/gam./day

* 3 months after the excavation.

Data on gamergate longevity under laboratory conditions:were obtained from the study of a small colony kept since November 1982i "Cocoons had been produced annually in this colony, and eggs were again laid at the end of 1985. These all developed into workers; there were never any males. In January 1986, dissection of all the surviving timid workers (n= 16) revealed that there was still one mated worker. All the unmated workers dissected had the ovarian.characteristics typical of infertile workers.

DISCUSSION

Most of the workers in the permanently queenless ant Rhytidoponera sp. 12 have numerous yolky oocytes in their ovaries. However, examination of the details of oocyte:growth, indicates that only mated individuals re- produce. The oocytes, in unmated workers, develop abnormally and do not mature. Since they, accumulate in the ovarioles~ yellow bodies are almost non-existent. Further to the evidence gathered duririg ovarian dissections, the absence of fresh eggs in laboratory: groups of unmated workers (all showing ovarian activity) confirmed that they do not lay.-Insemination thus changes the nature of the process of oocyteproduction, and leads to active egg-laying. A regular :association between insemination and reproductive activity has also been documented in Ophthalmopone berthoudi, O. hotten- DIVISION OF LABOUR IN RHYTIDOPONERA 83

tota and Pachycondyla krugeri (PEETERS and CREWE, 1985a, 1985b, 1986). In P. krugeri, unmated workers produce yolky oocytes of various sizes, but these do not reach more than half the size of an egg. In O. Berthoudi, ooge- nesis is restricted to mated workers, and their unmated nestmates generally have completely undeveloped ovaries. One can then categorically state that haploid and diploid eggs are laid exclusively by the gamergates. In R. sp. 12, an identical conclusion is based on circumstantial evidence from the pattern of ovarian development, and is supported by the observation that males were never produced in the laboratory. PAMILO et al. (1985) interpreted the occurrence of oocytes in mated and unmated workers to mean that they both lay eggs, a conclusion at variance with that in this study. PAMILO et al. determined the reproductive status of individual workers by counting the number of ovarioles which lack oocytes (" empty ovarioles "), and then suggested that mated workers have inter- mediate levels of ovarian activity. However, once the significance of empty ovarioles is appreciated, the data of PAMILO et al. (1985) support the findings of this study. Thus, unmated workers have more oocytes in their ovaries ("no empty ovarioles"), and mated workers show no accumulation of oocytes in their ovaries (" some empty ovarioles, and an intermediate number of oocytes per ovariole"). Some of the unmated workers collected by " fishing" also had empty ovarioles (PAMILO et al., 1985), but in this case it seems a consequence of resorbtion. Indeed, foragers dissected in the present study had ovaries with few or no oocytes. An association between ovarian state and behavioural activity is a general phenomenon in ants (reviewed by PASSERA, 1984: 189-194), and has been demonstrated for unmated queens as well as workers in Pachycondyla (= Neoponera) obscuricornis (FRESNEAU, 1984). An effect of oocyte production in unmated workers of R. sp. 12 is the storage of energy, and thus resorbtion can occur at times of the year when trophic conditions are unfavourable or when workers become active outside the nests. Dissection of a small sample (n = 20) of unmated workers col- lected underground in April 1986 showed that they had fewer and smaller oocytes in their ovaries than similar workers collected in October. PAMILO et al. (1985) suggested that in R. sp. 12, the unfertilized oocytes are laid as trophic eggs. Although trophic eggs are common in other ant subfamilies, they have only been reported in the following ponerine species : Pachycondyla obscuricornis "(FRESNEAU, 1984), P. apicalis, Ectatomma ruidum and E. tuberculatum (LACHAUD and FRESNEAU, pets. comm.). Oophagy has never been observed in R. sp. 12, and all eggs collected in the nests were similar in size (n = 17; length = 1,11 mm ___ 0,03 SD; width = 0,77 mm ___ 0,03 SD). Although the basal oocytes found in unmated workers display some of the characteristics of trophic eggs (e.g. heterogeneous yolk, flaccid structure due to thin or absent chorion, shape different to reproductive eggs, premature degeneracy of trophocytes; PASSERA et al., 1968, PASSERA, 1984: 84 C.P. PEETERS

50-51), their accumulation in the ovaries and the absence of yellow bodies indicate that they may never be laid. In many ant species, trophic eggs are a by-product of a regulatory act that inhibits male production, e.g. in Oecophylla longinoda, many workers exhibit ovarian activity, but if the queen is present, oocytes develop into trophic eggs only (H6LLDOBLER and WILSON, 1983). In R. sp. 12, the restriction of egg-laying to a small subset of workers is not the result of an inhibitory effect. Twenty-one gamergates were-found during the dissection of 2% workers from a colony excavated in October 1985. This represents 7,1% of the sample dissected, but this sample was biassed towards including gamergates. A more realistic percentage would be slightly lower for the following reasons: (1) only some of the marked foragers and other " above ground" ants were dissected (table I). This category of workers invariably lacked mature oocytes in their ovaries, and were all unmated ; (2) although eggs and gamer- gates were found in the random sample from chamber 1, no eggs were laid by the remaining workers. This suggests that there were no gamergates left, contrary to an expectation based on the results from the random sam- ple; (3) eggs were not found in the laboratory group of workers from the shaft chamber. Thus it is likely that most of the colony members that were not dissected would have been infertile. The numbers of inseminated workers reported in various queenless species of Rhytidoponera (e.g. WHELDEN, 1957; HASKINS and WHELDEN, 1965; WARD, 1983 ; PAMILO et al., 1985 ; this study) are relatively low compared with the situation in O. berthoudi, e.g. 125 gamergates were found in a colony with 311 workers (PEETERS and CREWE, 1985a). Unlike the Rhytidoponera Species studied, colonies of O. berthoudi were sampled throughout the year, and substantial variations in the ratio of gamergates to infertile workers were recorded. This ratio reaches a maximum following the period of male activity, and then decreases with the emergence of new workers. This is also evident in R. sp. 12, where a colony (November, 1983) with 437 adults yielded 23 gamergates (= 5,3 %) (PAMILO et al., 1985). Since there'were 138 callow workers in this colony, if it had been sampled a few weeks earlier (before the cocoons eclosed), gamergates would then have made up 7,7 % of the adult population. Indeed, the nature of the process of gamergate differentiation implies that there is no species-specific ratio of gamergates to infertile workers in a colony. This breeding characteristic is. determined by factors unique to each colony, e.g. the number of foreign males which locate a nest, and the number of young workers present underground. There is no" evidence that the number of gamergates in a col(,ny is under social regulation. In this colony of R. sp. 12, many gamergates occurred together in the same chambers (table I), showing that they tolerate each other. Worker reproduction in the Ponerinae is a eusocial alternative to queen- right breeding. ,Its study is still at an: early stag/" but it can be noted-that DIVISION OF LABOUR IN RHYTIDOPONERA 85 the association between insemination and reproductive activity has now been investigated in several species belonging to two ponerine tribes, and it remains to be falsified. Worker breeding as a reproductive strategy is pos- sible because there are many individuals that lay eggs in each colony. Ga- mergates lack the morphological specialization which is characteristic of a reproductive caste, and thus they have a relatively slow rate of egg-laying. The optimal number of gamergates per colony would be determined by the following mechanistic consideration. Ponerine workers in various species and tribes possess physiological traits which make them more or less capable of producing eggs quickly ; this ability is also a function of the number and length of ovarioles, and of the size of the eggs (which varies considerably in species with and without the queen caste; PEETERS, unpublished). Thus colonies of Rhytidoponera with a few gamergates will reproduce adequately since the latter have a relatively-high rate of egg-laying, while in O. berthoudi, many gamergates (laying larger eggs more slowly) may be essential for colonies to survive. However, at a proximate level, the average number of gamergates per colony is not directly inflttenced by the reproductive capa- bilities of workers. Gamergate differentiation is controlled by the number of males which disperse successfully in the population, notwithstanding how many workers of a suitable age are present in the nests. If in queenless species, the ineffectiveness of the gamergates is a constraint which limits breeding success, the only evolutionary option may be to alter the investment in male production.

ACKNOWLEDGMENTS. -- I thank Ross CROZIER for his help during the nest excavation, Rick TAYLOR from Fowler's Gap Research Station for the expert use of the Bako, and Ching CROZIER for maintaining one of the laboratory colonies for over two years. I am grateful to Drs L. PASSERA and R.H. CROZIER for their critical reading of the manuscript. This work was supported by an ARGS grant to R.H. CROZIER.

References

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