Honeybees (Apis mellifera) have haplodiploid sex determination:males develop from unfertilized eggs and females develop from fertilized ones.The differences in larval food also determine the development of females.Here we compared the total somatic gene expression profiles of 2-day and 4-day-old drone,queen and worker larvae by RNA-Seq.The results from a co-expression network analysis on all expressed genes showed that 2-day-old drone and worker larvae were closer in gene expression profiles than 2-day-old queen larvae.This indicated that for young larvae (2-day-old) environmental factors such as larval diet have a greater effect on gene expression profiles than ploidy or sex determination.Drones had the most distinct gene expression profiles at the 4-day larval stage,suggesting that haploidy,or sex dramatically affects the gene expression of honeybee larvae.Drone larvae showed fewer differences in gene expression profiles at the 2-day and 4-day time points than the worker and queen larval comparisons (598 against 1190 and 1181),suggesting a different patte of gene expression regulation during the larval development of haploid males compared to diploid females.This study indicates that early in development the queen caste has the most distinct gene expression profile,perhaps reflecting the very rapid growth and morphological specialization of this caste compared to workers and drones.Later in development the haploid male drones have the most distinct gene expression profile,perhaps reflecting the influence of ploidy or sex determination on gene expression.