Phenotypic integration in an extended phenotype: among-individual variation in nest-building traits of the alfalfa leafcutting bee (Megachile rotundata)

Authors: ROYAUTE, RAPHAEL - North Dakota State University; WILSON, ELISABETH - North Dakota State University; HELM, BRYAN - North Dakota State University; Mallinger, Rachel; Prasifka, Jarrad; GREENLEE, KENDRA - North Dakota State University; BOWSHER, JULIA - North Dakota State University

Submitted to: Journal of Evolutionary Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/23/2018
Publication Date: 3/2/2018
Citation: Royaute, R., Wilson, E.S., Helm, B.R., Mallinger, R.E., Prasifka, J.R., Greenlee, K.J., Bowsher, J.H. 2018. Phenotypic integration in an extended phenotype: among-individual variation in nest-building traits of the alfalfa leafcutting bee (Megachile rotundata). Journal of Evolutionary Biology. https://doi.org/10.1111/jeb.13259.
DOI: https://doi.org/10.1111/jeb.13259

Interpretive Summary: The alfalfa leafcutting bee (ALB) is an important managed pollinator for alfalfa and other crops, including blueberries, canola (oilseed rape), and native legumes. As solitary bees, female ALB create nests for their offspring. Nests are comprised of several cells. The structure of each cell is made from many pieces of leaf tissue, and inside the cell are pollen and nectar for food, along with a single egg which will develop into a male or female bee. From a large population of ALB, we collected bee nests and cells to determine whether (1) nest-building behavior varied among females, (2) the overall behavior was composed of groups of related nest traits, and (3) whether increases in one group of traits meant a decrease or trade-off for another group of traits. Female ALB did show individual differences in nest building. As females added cells to a nest, the later cells tended to be smaller. Various traits related to nest building were related, but only loosely and with little evidence for trade-offs. The results suggest that (naturally or artificially) ALB could be selected for a specific trait, like more cells per nest, without a large effect on other aspects of nest construction.

Technical Abstract: Structures such as nests and burrows are an essential component of many organisms’ life-cycle and requires a complex sequence of behaviors. Because behaviors can vary consistently among individuals and be correlated with one another, we hypothesized that these structures would 1) show evidence of among-individual variation, 2) be organized into distinct functional modules, and 3) show evidence of trade-offs among functional modules due to limits on energy budgets. We tested these hypotheses using the alfalfa leafcutting bee, Megachile rotundata, a solitary bee and important crop pollinator. M. rotundata constructs complex nests by gathering leaf materials to form a linear series of cells in pre-existing cavities. In this study, we examined variation in the following nest construction traits: reproduction (number of cells per nest and nest length), nest protection (cap length and number of leaves per cap), cell construction (cell size and number of leaves per cell), and cell provisioning (cell mass) from 60 nests. We found a general decline in investment in cell construction and provisioning with each new cell built. In addition, we found evidence for both repeatability and plasticity in cell provisioning with little evidence for trade-offs among traits. Instead, most traits were positively, albeit weakly, correlated (r ~ 0.15), and traits were loosely organized into covarying modules. Our results show that individual differences in nest construction are detectable at a level similar to that of other behavioral traits and that these traits are only weakly integrated. This suggests that nest components are capable of independent evolutionary trajectories.