Behav Ecol Sociobiol (1983) 12:1-4 Behavioral Ecology and Sociobiology �9 Springer-Verlag 1983

Intra-Colonial Relatedness Amongst Workers in a Population of Nests of the Polygynous Ant, Myrmica rubra Latreille

Barrie Pearson Institute of Terrestrial Ecology, Furzebrook Research Station, Wareham, Dorset BH20 5AS, England

Received December 1, 1981 / Accepted December 10, 1982

Summary. Intra-colonial levels o f genetic related- ness amongs t workers in the mult i -queened ant col- onies o f two popula t ions o f Myrmica rubra were found to be no t significantly different f rom zero with the exception o f one popula t ion in one o f the two years during which sampling occurred. It was tentatively concluded that worker relatedness varied f rom one site to another. Some conse- quences for mult i -queened colonies o f low related- ness are discussed.

Introduction

Hami l ton (1964) was the first to establish that the min imum condi t ion for coopera t ion between indi- viduals A and B in terms o f A and B maximising their inclusive fitness is K > l/taB where K is the ratio o f the gain by the beneficiary to the loss o f the donor in terms of offspring produced, and raB is the pedigree coefficient of relatedness between A and B. The pedigree coefficient o f relatedness between A and B is the p ropor t ion o f A ' s genes identical by descent to those present in B. It is apparent that the higher the value rAB , the more readily coopera t ion m ay occur. Coefficients o f re- latedness have been measured in a number o f social insects (e.g., Metca l f and Whit t 1977; Pamilo and Varv io -Aho 1979; Craig and Crozier 1979; Pearson 1982; Lester and Selander, in prepara- tion), using the alMic determinants o f certain isoenzyme systems as the basis o f these estimates. All these studies have been concerned with queen coopera t ion in multi-queened, ie polygynic, species. N P Pearson 1982 measured relatedness amongs t queens in the po lygynous ant species

Myrmica rubra; queen relatedness was variable be- tween years and popula t ions and is not apparent ly a pre-requisite for cooperat ion. This present s tudy measures relatedness amongs t workers in a colony in the same populat ions. It does not necessarily follow that workers will have the same levels o f relatedness as queens, especially where only one or a small percentage o f queens contr ibute towards the worker force (non-funct ional polygyny). Con- sistently high levels o f worker relatedness in these popula t ions would be commensura te with non- functional po lygyny and could explain po lygyny in this species.

Materials and Methods

The sampling method has been described in Pearson and Child (1980) and Pearson (1982). Two sites in South Dorset, England, which will be referred to as A and B, were sampled in 1977 and 1978, and, in addition, data from samples taken at Site A in 1975 and at Site B in 1975 and 1976 were used. Both sites are on limestone grassland. Site A had a high density of nest centres and covered a few square metres. A nest is defined here as any stone used regularly by members of a colony; stones have a heat storage capacity making them attrac- tive to ants as nest centres. Some nest centres at Site A and Site B had, some years previously, been excavated and appeared to be discrete at both sites, despite the close proximity of nest centres to each other at Site A. It seems, therefore, that neither the population at Site A, nor that at Site B, was a polydomous colony. The Site A population was relatively isolated from other populations of M. rubra. Site B covered a larger area - about a hectare - and had a lower density of nests and was less isolated from other M. rubra populations.

The electrophoretic method was as described in Pearson and Child (1980). The alleles postulated as the determinants of the esterases are labelled F and S, giving the genotypes FF, FS and SS (Pearson and Child 1980). The twoallele hypothesis was tested by examining 432 males (Pearson and Child 1980) where no heterozygotes were found. This is consistent with the two-allele hypothesis, as the males are haploid.

b, the regression coefficient of relatedness, was estimated using the method of Pamilo and Crozier (1982), a method de- rived from Orlove (1975). This is the method used by Pamilo and Varvio-Aho (1979) to estimate b in colonies of Formica sanguinea amongst workers and males and by Pearson (1982) to measure intra-colonial relatedness amongst M. rubra queens. This method estimates the regression coefficient (b) of the fre- quency of an allele in each individual in a nest, plotted against the mean frequency of that allele for the whole nest for a popu- lation of nests. When there is no selection or inbreeding, esti- mates of b fall close to the pedigree estimates (Pamilo and Crozier 1982).

individual per nest was sampled and the possibility that the worker population of each nest may not represent a random sample of the gametes in the population as a whole. However, in those nests where agreement does occur with the expected Hardy-Weinberg ratio, it would seem more prob- able that the workers do represent the products of a random sample of gametes. The alternative explanation that chance alone was responsible for the agreement seems unlikely, despite the fact that

Results

1887 workers were studied by electrophoresis. Ex- pected Hardy-Weinberg (with Levene's correction) values agree with the observed values at Site A for 1975, 1977 and 1978, and at Site B for 1977 (Table 1). Site B genotype data for 1975, 1976 and 1978 were not included in Table 2 since b values here may be subject to selection or inbreeding effects. The Hardy-Weinberg test is of limited value here because of the possibility of the non- random sampling of genotypes, as more than one

Table 1. Worker genotype totals for all sites examined, and significance of the difference between observed and expected (based on Hardy Weinberg ratios) genotypes estimated by Z 2 (with Levene's correction)

Site Year FF FS SS P

Table 2. Distribution of worker genotypes in individual nests

Year Worker genotypes

FF FS SS FF FS SS

SiteA 1975 2 14 25 0 4 36 0 3 37 1 14 23 0 8 35 1 16 25 0 1 17

Si teA 1977 2 11 28 1 29 13 Ske B 1977 6 14 20 44 16 0 1 15 21 41 0 0 0 13 29 35 5 2

12 28 0 22 18 0 33 7 0 40 0 0

0 36 0 A 1975 4 60 198 >0.05 B 1975 504 95 44 <0.001 Site A 1978 0 0 6 B 1976 82 28 24 <0.001 0 7 20 A 1977 9 53 98 >0.05 0 6 8 B 1977 228 139 15 >0.05 3 9 18 A 1978 4 23 75 >0.05 1 1 2 B 1978 84 36 11 ~0.02 0 0 21

Table 3. b-values for intra-nest relatedness between workers and queens (queen data from Pearson 1982), with t-test estimate of significance

Site Year b 95% confidence limits Number of colonies P

w Q w Q w Q w Q

1975 0.1056 -0 .1962 7 0.4074

A [1977 0.0218 -0 .1487 -0 .2761 -0 .6156 4 4 0.3197 0.3182

B 1977 0.5428 0.3656 0.1665 -0 .2723 9 6 0.9191 1.0035

A 1978 0.0828 0.1705 -0 .4072 -0 .2836 6 6 0.5728 0.6246

B 1978 0.6721 0.3032 11 1.0410

>0.05

>0.05

0.02

>0.05

>0.05

>0.05

>0.05

<0.01

the workers were not actually the product of a random sample of the populations' gametes.

In conclusion, it is conceded that use of Hardy- Weinberg, for this purpose, in the workers is con- troversial. Pamilo and Varvio-Aho (1979) did not use this method, tacitly assuming selection was not occurring. However, Craig and Crozier (1979) did apply the test. On the basis that its application is more likely to err towards the elimination of data where there are no inbreeding or selection effects, rather than not eliminating data where these effects occur, the use of the test is considered conservative.

Genotype data for nests are given in Table 2. b values and their significance levels, together with 95% confidence limits, are given in Table 3, to- gether with the data for the queens from Pearson (1982). From Table 3 it can be seen that related- ness amongst both queens and workers is variable through time and between sites. There is a signifi- cant level of relatedness at Site B amongst workers in 1977, and at Site B in 1978 amongst queens. In all instances, the range of b values is large, and the 95% confidence level intervals are wide (Ta- ble 3). This is most probably due to small sample sizes.

Discussion

The data in Table 3 suggest that the levels of worker relatedness, like those of queens in these populations of M. rubra (Pearson 1982), may not be high. The exceptions are the worker population 1977B, and queen population 1978B, both of which had high b values which were significantly different from zero. The 95% confidence limits include a wide range of possible values for b in each population of each year, encouraging a cau- tious interpretation of this data with regard to in- tracolonial relatedness in the workers (and queens). The finding of levels of relatedness not significantly different from zero amongst the workers in colonies of M. rubra is similar to Craig and Crozier's (1979) findings in Myrmeciapilosula, a species also thought to be functionally polygyn- ous. In that study, workers were found to have b values significantly different from zero, using a less conservative estimate of degrees of freedom than that developed by Pamilo and Crozier (1982). When worker relatedness in M. pilosula was esti- mated by the method used here, it was found to be not significant (P>0.05). Pamilo and Varvio- Aho (1979), in contrast, found significant and high levels of relatedness amongst the workers of a pop- ulation of Formica sanguinea. The conclusion

which may be tentatively drawn is that worker re- latedness in ant species exhibiting secondary poly- gyny may vary from species to species and, from the data here, possibly from population to popula- tion, and year to year. The non-significant b values in the workers of M. rubra reinforce the impression given by the b values for gynes (Pearson 1982) that high relatedness may not be important in establish- ing polygyny in this species.

Low levels of relatedness between the workers in M. rubra may be attributed to the involvement of more than one queen in worker production (low relatedness amongst the queens in this instance would also lower relatedness amongst the workers), multiple inseminations, Or both. The genotype data of Table 2 indicate that in many nests there may be more than one laying queen, multiple inseminations, or both. A system where queens and workers of low relatedness could inter- relate in the manner outlined above is through an association of queens, at least originally, derived mutualistically. This could occur if the conditions of nest foundation, the cause of temporary poly- gyny, persisted. Habitat ephemerality mimics this condition. If the fluctuation of key habitat vari- ables was such that worker and queen mortality was frequently high, a rapid production of workers would be required to maximise reproductive suc- cess when conditions became favourable, ie surplus of reproductive potential is required to track a varying environment. Elmes (1976) found that there are invariably more queens than the detected reproductive effort requires in colonies of M. rubra. If queens are recruited within the colony to act as a buffer against variability, intra-colonial relatedness may be increased; evidence for this is discussed by Pearson (1982). Relatedness, there- fore, may be a consequuence of mutualism and habitat variability in M. rubra, and eventually could be a stabilising influence on colony interac- tions. This may be the basis of a more general explanation for the derivation of eusociality from the primitive associations of some semi-social, hy- menopteran precursors of the eusocial state. The observed persistence of colonies of M. rubra would seem to be counter to this hypothesis, but this may be a misleading measure of their habitat stability since high mortality of individuals in the colony may be concealed by queen recruitment and a rapid turnover of workers serviced by those rec- ruits.

Acknowledgements. I would especially like to thank R.H. Crozier, Pekka Pamilo for many useful suggestions in preparing this paper, and also M.V. Brian, R. Clarke, J.E. Pearson and M.K. Perkins.

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