GENOTYPIC WITHIN-COLONY INTERACTIONS AND THE COLONY DEFENSE IN APIS MELLIFERA

Grzegorz Borsuk, Jerzy Paleolog
Department of Biological Basis of Animal Production, Agricultural University in Lublin, Poland

ABSTRACT

The authors studied how genotypic variability within colonies affects their defensive response. Different genetic types of bees were used in the experiments. The influence of the artificially generated within-colony genotypic diversity on the colony defence was investigated. Common stinging assays were performed. Time to the first sting was (TFS) and the number of stings made to the leather target within 2 minutes (NS) were recorded. The contribution of both defensive and gentle workers in the defensive response of the colonies being a physical mix of two such worker types was also studied (film analysis of the bees attacking the target). It was concluded that the within-colony, inter-worker genotypic interactions influenced the colony defense response due to the fact that the quantified value of the colony was usually not the additive composite of diverse worker groups of which the colony was composed. Moreover, the inter-worker interactions were dependent both on the quantified characteristics, which were different for TFS and NS, and on the types of the combined bees. The film analysis revealed that the contribution of defensive/gentle bees to the colony defense only partly reflect the defensive : gentle worker ratio in the colonies composed of such worker types. The contribution also depended on the types of mixed workers.

Key words: Honeybee, defensive behaviour, genotypic-interactions, mixed-colony, sociobiology.

INTRODUCTION

In honey bees, unlike other farm animals, phenotype/genotype of the evaluated trait should be considered not only with regard to a single individual but at the colony level as well. Most features of economic importance also result from the behaviour of the whole colony. Because of the polyandry [6] the bee colony consists of worker groups having different genotypes/phenotypes [8, 10]. In beekeeping the worker diversity can be increased when brood is exchanged between colonies, when colonies are combined and, last but not least by bee drifting [1, 12]. Hence, both genetic and genotypic variance should be considered in breeding programs. Genetic variance brings about variability of colony mean genotypes, whereas the within-colony genotypic variance could cause nonadditive interaction effects related to the specific genotypic mix of workers in the colony [8]. If the colony value is the additive composite of diverse worker groups, the colony mean would be the average of these groups. If the nonadditive interactions occur, this mean should be similar/superior/inferior to the value of one of these groups. It was revealed [9] that the genetically determined behaviour of a single worker or a worker group could be modified by the behaviour of the another worker group. Thus, the genotypic variance within a colony could affect several colony traits [8, 10], and therefore, could also affect the estimated colony value.

Defensive behaviour is an important colony characteristic particularly in the densely populated Europe, where bees are kept in the proximity of human residences. Moreover, work with gentle bees is easier and less laborious, both in small and in big commercial bee yards. Therefore, the authors decided to determine how genotypic variability within colonies could affect their defensive response in order to help better evaluate that trait in breeding practice [4]. Hence, the present research concerned the influence of the artificially generated within-colony genotypic diversity on the colony defense and particularly the contribution of defensive and gentle workers in the defensive response of those colonies which were a physical mix of two such worker types.

MATERIALS AND METHODS

In order to acquire four genetically different groups of workers of the balanced age structure, bees from six different colonies were sampled after the sunset. These were: gentle Italian bees (GI), two groups of gentle Buckfast bees (GB₁, GB₂), highly defensive carniolan crossbreeds (DC) and two groups of highly defensive native bees (DN₁, DN₂). Workers of only two of these groups were used in each of the four conducted comparisons (hereafter referred to as C1, C2, C3, C4) in order to set up homogenous and mixed nuclei colonies that were of the same strength and structure [13]. Each of the nuclei colony consisted of 2 liters of bees, 2 Langstroth combs with honey and pollen stores, 2 Langstroth combs with open brood and a young egg laying queen. Such small nuclei colonies are believed to be fully functional. In C1 the homogenous nuclei colonies 100%DN₁ and 100%GI were compared with a nuclei colony that was a physical mix of 50% DN₁ and 50% GI bees (50%DN₁-50%GI). In C2 the comparison comprised the homogenous nuclei colonies 100%DN₂, 100%GI and a mixed 50%DN₂-50%GI, in C3 the homogenous nuclei colonies 100%DN₁, 100%GB₁ and mixed 20%DN₁-80%GB₁, 50%DN₁-50%GB₁, 80%DN₁-20%GB₁ and finally, in C4 homogenous nuclei colonies 100%DC, 100%GB₂ and a mixed 50%DC-50% GB₂. The common stinging assay was carried out [7, 11] in order to quantify the defensive response of mixed and homogenous nuclei colonies. The dark, spherical, leather target of the surface of 360cm² was waved in front of the entrance of each hive after the colony had been disturbed by knocking. Separate targets were used for separate colonies. The time at which the first sting was made into the target (TFS) and the number of stings received by it (NS) within 2 minutes after TFS, were recorded. During 15 consecutive days, 15 repetitions of this assay were carried out in the case of each of the three comparisons (C1, C2, C3). In the case of C4 20 repetitions were performed. Thus the data base of "3 nuclei colonies types * 15 repetitions" was obtained in the case of C1 and C2 and "3 nuclei colonies types * 20 repetitions" in C4, whereas in the case of C3 it was "5 nuclei colonies types * 15 repetitions". One way ANOVA, homogeneity of variance test and Duncan multiple range test were performed separately for C1, C2, C3 and C4. The emerging brood was removed over the stinging assay period to maintain the genotypic structure of the examined nuclei colonies. Additionally, dark-coloured defensive and light-coloured gentle bees were recorded on the film. This difference allows us to distinguish the defensive and the gentle bees on this film when they were flying in the space around the target and the n, to count them once every 15 seconds over the 2-minute period after TFS over the stinging assay.

RESULTS AND DISCUSSION

In the nuclei colonies (further referred to as colonies) mixed of gentle and defensive bees (50%), the value of TFS was similar to that observed in the colonies composed of 100% defensive bees in C1, C2, C3 and C4 (Table 1). NS of the mixed colony (50%) was intermediate only in C3. In C1 NS was more similar to that observed in 100% defensive colony, whereas in C2 and C4 it was more similar to that in 100% gentle colony. Admixture of 20% defensive bees (C3) did not increase the defense of the gentle bees in terms of TFS and NS. Admixture of 20% gentle bees did not decrease the defense response of the defensive bees in terms of TFS, but increased it, quite unexpectedly, in terms of NS. This tendency did not found the statistical confirmation because of high variability. Hence, the examined bees did not combine additively [8] and the defensive bees showed behavioural dominance [4] regardless of the genotypes of mixed workers, but only in terms of TFS. This finding corresponds with the results of DeGrandi-Hoffman at al [3]. In the case of NS, however, the inter-worker interactions were more specific, because depending on the genotypes of the mixed workers either the defensive or the gentle bees dominated or they combined almost additively. In 80%DN₁-20%GB₁ something like "over-domination" may even be suggested. Consequently, worker inter-genotypic interactions changed the value of the colony mean phenotype but in different ways in the case of TFS and NS. It have been supposed that TFS and NS could be determined by different genes [3]. However, the results of the present study revealed that this negative genetic correlation could also result in different nature of the inter-genotypic worker interactions observed in the case of these two traits. Variability of the examined traits expressed as CV was not higher in the mixed colonies than one could expect. No relationships between the colony composition and the CV values were observed.

Our results show that the quantified value of the colony defense response is usually not the additive composite of diverse worker groups of which the colony is composed. Therefore, it was interesting to find out whether the contribution of the gentle/defensive workers to the colony defense response was similar to the worker ratio in the mixed colony (gentle : defensive). The film analysis (Table 2) revealed that the contribution of defensive bees to the defense response of the colonies that were mixed of 50% defensive and 50% gentle bees was greater than 50% in C1, C2 and C4, whereas that contribution was about 50% (do not differ from the expected value for the additive inter-worker interactions) in C3. Such contribution was greater in 80%DN₁-20%GB₁ than in 20%DN₁-80%GB₁ but the difference did not reflect the defensive : gentle worker ratio. The gentle worker dominated in the case of 80%DN₁-20%GB₁ and there were the defensive workers in the case of 20%DN₁-80%GB₁. Consequently, results of the common stinging assay only partly reflected the complex defense behaviour, since some individuals reacted (flew out the hive) but did not sting the target. This finding is in agreement with some other works [2, 5]. Consequently, the so called "out-flyer rate" that was measured in some studies of the bee defensive behaviour [5] does not always need to be in agreement with the results of the common stinging test [7, 11]. Additionally, one should take it into consideration that the defense strategies in western honeybees are connected not only with aggressiveness but also with docility [5]. Therefore, more studies with the film techniques are necessary in order to learn about defensive behaviour of European bees and particularly, about the inter-worker genotypic interactions.

CONCLUSIONS

1. Within-colony inter-worker genotypic interactions exerted a considerable influence on the colony defense response. The interactions were dependent both on the quantified characteristics (time to the first sting / the sting number) and on the type of the combined bees.

2. Different defensive and different gentle bees could combine differently in mixed colonies.

3. The common stinging assay only partly reflects the complex defense behavior. In further research the film analysis of the out-flying bees seems to be useful.

ACKNOWLEDGEMENT

Researches was founded by State Committee for Scientific Research as a part of the scientific project: 2 P06D 008 26 2004/05.

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