HETEROSIS IN THE HONEY BEE (APIS MELLIFERA L.)

Abstract

HERE are three castes in the honey bee colony: the drone, queen, and worker. T The latter two are both females. The drone bee is a male, representing a haploid gamete of the queen bee heading his hive. The honey bee colony is composed of two generations of bees. The queen bee is, in effect, a potent egg-laying machine, while the workers are morphologically and functionally fitted for honey and pollen gather-ing and for most other work of hive and field, except that under normal circumstances they do not lay eggs. The queen bee is the parent mother of the colony, and the worker bees are her progeny. Breeding methods to insure heterosis have led to increased vigor and productivity in corn, sorghum, Drosophila, poultry and other species. These species are diploid in both sexes. It is of interest to examine another species, having a different system of reproduction, for heterotic effects and to analyze for the responsible mechanisms. In this study hybrid vigor is regarded as the superiority of the hybrid over the better parent. The characters chosen for study are egg production of the queen, a character largely dependent on the queen's own genotype, and honey yield, a character depend-ent on the vigor and well being of the queen's worker progeny. Honey yield conse-quently differs from egg production in being influenced by the genome of the drones to which the queen is mated. The single queen will contribute a genome to each of the 50 or 60 thousand workers making up the colony. In the past the honey bees must have been intensely selected for the capacity to produce large numbers of progeny and workers active in gathering food supplies. The individual worker bee lives about 6 weeks during the active summer season. Rapid replacement of this worker force depends on the egg laying of the single queen mother. For existence the colony must harvest sufficient nectar in six to ten weeks to last the balance of the year. Till recently, numbers for egg laying and colony replacements have been rather vague, 200 to 5000 eggs per day were estimated. NOLAN (1923) has presented data indicating that rates of less than 2000 eggs per day over a 12 day period were to be expected. Larval deaths will reduce the effective worker groups. The effective oviposition rate will be less than the observed eggs laid, the actual oviposition, by any genetic or other factors favoring the death of the larva. In the inbreeding necessary to the formation of the inbred lines utilized in this study hypothesized sex alleles could be factors in this loss of larvae. MACKENSEN (1951), using brood viability counts, has postulated for the honey bees a series of haplo-viable alleles which are lethal in homozygous diploid females. However, earlier work of CALE (1952) indicated that the oviposition rate of inbred queens was independent No. 1187. Journal Paper No. 5-2816 of the Iowa Agricultural Experiment Station, Ames, Iowa. Project HETEROSIS IN THE HONEY BEE 293 of percentage inbreeding in the range 31 to 77 percent. The queens utilized in this study were observed to have high egg viability prior to collection of the data. MATERIALS AND METHODS The genetic material for this study was composed of four inbred lines of honey bees. Following an original selection, no further selections were practiced during the inbreeding of any of these lines. No precise method of inbreeding was used in establishing and continuing any of the lines. Selection of male and female parents from one generation to the next within each strain was more often determined by the availability of males than by attempting to retain a strict parent-offspring or brother-sister system of inbreeding. The coefficients of inbreeding for the four lines a t the initiation of these experiments were as follows: D, 56.7 %; F, 54.6%; H, 65.8%; J, 64.8%. All matings in the formation of these inbred lines were instrumental