Nutritional Regulation of Phenotypic Plasticity in a Solitary Bee (Hymenoptera: Megachilidae)

Abstract

Phenotypic plasticity involves adaptive responses to predictable environmental fluctuations and may promote evolutionary change. We studied the regulation of phenotypic plasticity in an important agricultural pollinator, the solitary alfalfa leafcutting bee (Megachile rotundata F.). Specifically, we investigated how larval nutrition affects M. rotundata diapause plasticity and how diapause plasticity affects adult female reproductive behavior. Field surveys and laboratory manipulations of aspects of larval diet demonstrated nutritional regulation of M. rotundata diapause plasticity. Manipulation of larval diet quality through the addition of royal jelly, the caste-determining substance of the honey bee Apis mellifera L., increased the probability of diapause in M. rotundata. We also found that larval nutrition and diapause status affected M. rotundata adult female reproductive behavior. Nutritional effects on larval diapause that also impact adult fitness have intriguing implications for the evolution of developmental plasticity in bees. In particular, as the solitary lifestyle of M. rotundata is considered to be the ancestral condition in bees, nutritionally regulated plasticity may have been an ancestral condition in all bees that facilitated the evolution of other forms of phenotypic plasticity, such as the castes of social bees.

Keywords: developmental plasticity; diapause; nutrition.

Fig. 1.

Megachile rotundata provision size and adult body weight of diapausers and nondiapausers from field-collected nests. (A) Example of an x-radiograph of collected nests. Individual provisions were traced, and the area enclosed in the outline was calculated. (B) The relationship between provision size and diapause probability. Each line represents the logistic regression for one year and sex combination. (C) Adult body weight of male and female diapausers and nondiapausers from nests collected in Year 1 and Year 2. Boxes extend to upper and lower quartiles, line within box represents median, whiskers extend to 1.5 × interquartile distance (white boxes: Diapausers, gray boxes: Nondiapausers, **: Tukey HSD P < 0.001, ns: not significant). (D) The relationship between provision size and adult body weight. Each line represents the linear regression for one year and diapause status combination.

Fig. 2.

Effects of M. rotundata larval diet treatments on larval weight, adult weight, and diapause incidence. (A) Example of dish set up used for rearing of larvae on manipulated diets. (B) Weight of fifth-instar larvae reared on different diets. n: Number of larvae. (C, D) Weights of adult females (C) and adult males (D) reared on different diets. D/ND: Number of diapausers/nondiapausers weighed. Boxes extend to upper and lower quartiles, line within box represents median, whiskers extend to 1.5 × interquartile distance. (E) Probability of diapause of larvae reared on different diets. Bars represent mean ± standard error of mean. Groups with different letters have significantly different means (Tukey HSD P < 0.05). n: Sample size (number of rearing dishes).

Fig. 3.

Total cells built by M. rotundata females in noncompetitive nesting assays. In both Year 1 and Year 2, three cohorts of 20–25 age-matched females were released into separate field enclosures containing at least two nesting holes for each female. (A) Total cells built by diapausers [D] and nondiapausers [ND] in Year 1 and Year 2. Diapause status had a significant effect on total cells built in Year 1, but not in Year 2 (**: P < 0.001, ns: not significant). (B) The relationship between female adult weight and total cells built by diapausers [D] and nondiapausers [ND] in Year 1. Weight did not significantly affect total cells built (P > 0.05). (C) The relationship between female weight and total cells built by diapausers [D] and nondiapausers [ND] in Year 2. There was a statistically significant effect of weight on total cells built (P < 0.05); however, the pseudo-R² for the correlation was 0.064, indicating that weight explains very little of the variation in total cells built.

Fig. 4.

The relationship between the number of days an M. rotundata female controlled a nesting hole and her weight in competitive nesting assays. In Year 2, four cohorts of 18–24 age-matched females were released into separate field enclosures containing one nesting hole per two females. Data were pooled from the four replicates, and weights were standardized using z-scores to account for variation in the distributions of weights among replicates. There was a quadratic effect of weight on the number of days a female controlled a nesting hole (P < 0.001). D: diapausers, ND: nondiapausers.