The genetic basis of transgressive ovary size in honeybee workers

Abstract

Ovarioles are the functional unit of the female insect reproductive organs and the number of ovarioles per ovary strongly influences egg-laying rate and fecundity. Social evolution in the honeybee (Apis mellifera) has resulted in queens with 200-360 total ovarioles and workers with usually 20 or less. In addition, variation in ovariole number among workers relates to worker sensory tuning, foraging behavior, and the ability to lay unfertilized male-destined eggs. To study the genetic architecture of worker ovariole number, we performed a series of crosses between Africanized and European bees that differ in worker ovariole number. Unexpectedly, these crosses produced transgressive worker phenotypes with extreme ovariole numbers that were sensitive to the social environment. We used a new selective pooled DNA interval mapping approach with two Africanized backcrosses to identify quantitative trait loci (QTL) underlying the transgressive ovary phenotype. We identified one QTL on chromosome 11 and found some evidence for another QTL on chromosome 2. Both QTL regions contain plausible functional candidate genes. The ovariole number of foragers was correlated with the sugar concentration of collected nectar, supporting previous studies showing a link between worker physiology and foraging behavior. We discuss how the phenotype of extreme worker ovariole numbers and the underlying genetic factors we identified could be linked to the development of queen traits.

Figure 1.—

Crossing scheme used to analyze the genetic architecture of ovary size differences between Africanized and European honeybees. Inbred crosses are not shown for clarity.

Figure 2.—

Box and whisker plots of total worker ovariole number for the initial survey of 12 AHB and 12 EHB colonies. The AHB colony with the highest ovariole counts (A2) was used as the AHB parent and the EHB colony with the lowest ovariole counts (E10) was used as the EHB parent in subsequent crosses.

Figure 3.—

Box and whisker plots of total worker ovariole number for the crosses between AHB and EHB that revealed the extreme ovariole number phenotype. From left to right is the AHB parent (A; shown as colony A2 in Figure 2), inbred AHB (AI 1–4), backcross AHB (ABC 1–8), hybrid (ExA), backcross EHB (EBC 3–10), inbred EHB (EI 1–3), and the EHB parent (E; shown as colony E10 in Figure 2). AHB backcross colonies ABC3 and ABC5 with extreme ovariole number phenotype were used for selective pooled DNA QTL mapping.

Figure 4.—

Normal-sized worker ovary from the Africanized parent colony A (A) and large worker ovary from Africanized backcross colony ABC3 (B). Each visible filament is an ovariole (e.g., there are 7 ovarioles in A and 46 ovarioles in B).

Figure 5.—

(A) Histogram of ABC3 worker total ovariole number sampled in May and July, raised in natal colony; median 76, range 6–147. (B) Histogram of ABC5 worker total ovariole number sampled in May, raised in natal colony; median 30, range 3–94.

Figure 6.—

(A) Histogram of ABC3 worker total ovariole number used for selective pooled QTL mapping, raised in an unrelated nurse colony; median 23.5, range 5–62; N = 88. Shading indicates samples included in the high and low pools. (B) Histogram of ABC5 worker total ovariole number used for selective pooled QTL mapping, raised in an unrelated nurse colony; median 23.0, range 7–58; N = 571. Shading indicates samples included in the high and low pools.

Figure 7.—

LOD score values for combined ABC3 and ABC5 populations showing the location of a QTL on chromosome 2 with liberal marker selection. Triangles show marker location. The dotted line shows the 95% genomewide threshold for QTL detection and the dashed line the 99% genomewide threshold.

Figure 8.—

QTL on chromosome 11 detected with liberal marker selection. The horizontal black bar shows the 95% C.I. for a QTL affecting age at first foraging described in Rueppell (2009).

Figure 9.—

QTL on chromosome 11 detected with conservative marker selection.

Figure 10.—

The sugar concentration of nectar collected by foragers is negatively correlated with forager total ovariole number (R² = 0.084). Foragers from two Africanized backcross colonies (ABC5 and ABC8) were used.