Staying close to home

Highlighting the article written by A. Friedel, R. J. Paxton, A. Soro (2017) in Insectes Sociaux

Written by Insectes Sociaux Editor-in-Chief, Michael Breed (michael.breed@colorado.edu)

In this issue of Insectes Sociaux, Friedel et al. (2017) report on their investigations of population structure of a eusocial sweat bee, Lasioglossum malachurum. Their fascinating study exemplifies the complex choices that dispersing animals face. An animal that moves far from its birth location encounters unknown arrays of predators, uncertainty in finding a suitable destination habitat, and the possibility that all of the good habitat space in their path is already occupied. On the other hand, staying close to home raises the likelihood of competing with close relatives as well as inheriting any diseases or parasites that beset the previous generation.

For animals living in aggregations some challenges may be amplified. In addition to high potential for competition with close relatives, disease and parasite problems are compounded, as many possible hosts live in close proximity to one another. Aggregations may form via philopatry, in which animals establish nests close to their natal nest, which creates potential questions about social evolution. If more than one individual lives in a nest, as is the case in the closely related Lasioglossum zephyrum, Then the mechanisms of social recognition that maintain high levels of familial relationship within each nest may break down. High genetic similarity among nearby colonies, as in Lasioglossum malachurum, may blur kin distinctions, setting up possible difficulties for kin-selection explanations of the evolution of a worker caste. Given these intriguing contradictions between advantages in settling near the natal location and dispersing further, this investigation (Friedel et al. 2017) of genetic structure is particularly interesting and timely.

Friedel et al. (2017) specifically test the hypothesis that when gynes (potential queens) search for a new nesting site they are likely to choose a location near their natal nest. By using microsatellite markers to investigate genetic similarity they were able to determine that bees in neighboring nests have higher degrees of genetic similarity than bees from more distant nests within the same aggregation. They found in three of four aggregations studied that bees from very close nests were more genetically similar than expected if random dispersal had taken place. In other words, very short range dispersal seems to have resulted in the formation of aggregations of individuals or nests in small pockets within larger expanses of seemingly suitable habitat. Colonies located further apart within each aggregation showed random degrees of genetic similarity. The aggregation in which no population substructure was observed is very large and perhaps older, leading the authors to suggest that founder effects account for small scale genetic similarity within aggregations and that over time immigration of unrelated bees in the aggregation dilutes these effects. This study sets the stage for assessing how population genetic substructure affects social evolution and disease-host relationships.

The halictid bee studied by Friedel et al. (2017) is typical of many eusocial insects, which establish their nests in aggregations of tens, hundreds or even thousands of individual colonies. Aggregated nests are common in many species of halictid sweat bee, in some species of Apidae, such as Apis dorsata, and in some species of the social wasps Polistes, Mischocyttarus, and Ropalidia. We can also look beyond insects to find many interesting behavioral and evolutionary analogies that can be seen in the aggregated nesting of birds, such as oropendolas, Psarocolius spp., and various swallows Hirundo spp., and in mammals such as prairie dogs, Cynomys spp.

The repeated appearance of aggregated nesting across this wide range of taxa raises many interesting evolutionary and behavioral questions. On the surface, if aggregated nests result from limited dispersal from natal sites, this could have major effects on population genetic structure, leading to high levels of genetic similarity among individuals within small sampling areas in the aggregation. As Friedel et al. (2017) point out, this may represent a transitory phase in the development of aggregations across generational time, as long-term movement of unrelated animals into the aggregation may counterbalance the short-term effects of an aggregation having been initiated by a single female and her immediate offspring.

 Reference

A. Friedel, R. J. Paxton, A. Soro (2017) Spatial patterns of relatedness within nesting aggregations of the primitively eusocial sweat bee Lasioglossum malachurum. Insectes Sociaux DOI: 10.1007/s00040-017-0559-6

 

 

 

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