Honeybees (Apis mellifera) have one of the most fascinating sex determination systems — haplodiploidy. In this system, sex is determined by the ploidy level of the organism, not by sex chromosomes. Females are diploid (2n = 32 chromosomes), males are haploid (n = 16 chromosomes). Females develop from fertilised eggs (diploid). Males develop from unfertilised eggs by parthenogenesis (haploid). This system is found in all Hymenoptera (bees, wasps, ants) and some other insects. The queen controls sex of offspring by deciding whether to fertilise eggs: fertilised → female (queen or worker depending on nutrition/rearing); unfertilised → male drone.

Queen: diploid female (2n=32). Develops from fertilised egg reared in a large queen cell and fed exclusively on royal jelly (rich protein secretion from worker hypopharyngeal glands). One queen per colony. Sole reproductive female — lays up to 2000 eggs per day. Lives 3-5 years. During nuptial flight: mates with multiple drones (polyandry) → stores sperm in spermatheca → uses stored sperm to fertilise eggs throughout her life. Workers: diploid females (2n=32). Develop from fertilised eggs reared in smaller cells and fed royal jelly briefly, then bee bread (pollen + honey). Sterile under normal conditions (ovaries suppressed by queen pheromone). Perform all colony tasks: nursing larvae, wax secretion, guarding, foraging. Live 6 weeks (summer) to 6 months (winter). Drones: haploid males (n=16). Develop from unfertilised eggs (parthenogenesis). No stinger, do not forage. Sole function: mating with virgin queens during nuptial flight. Die immediately after mating (genitalia torn away). Expelled from hive before winter.

Meiosis requires diploid cells (2n) — it is a reductional division that halves the chromosome number from 2n to n. Drones are already haploid (n=16). Haploid cells cannot undergo meiosis (there is no homologous chromosome pair to separate in meiosis I). Therefore, drone honeybees produce sperm by MITOSIS — each mitotic division of a haploid (n) spermatocyte → two haploid (n) spermatids → mature sperm. This is why statement D (males produce sperm by meiosis) is FALSE. This is a critical distinction and common NEET trap. All resulting sperm from a drone are genetically IDENTICAL (no recombination possible — no meiosis, no crossing over). Compare: human males are diploid → spermatogenesis involves meiosis → genetically diverse sperm.

XX-XY system (humans, Drosophila): females XX, males XY. Sex chromosomes determine sex. Autosomes + sex chromosomes all present. XO system (grasshopper, Locusta): females XX, males XO. Male has one X, no Y. Females are diploid, males are diploid (but with one fewer sex chromosome). ZW-ZZ system (birds, some fish, butterflies): females ZW (heterogametic), males ZZ (homogametic). Opposite of mammals. Haplodiploid (bees, wasps, ants): sex determined by ploidy not sex chromosomes. Females diploid, males haploid. Temperature-dependent sex determination: in many reptiles (turtles, crocodiles, some lizards). No sex chromosomes — incubation temperature determines sex. Environmental sex determination: some fish (clownfish, wrasse) can change sex based on social environment — sequential hermaphroditism.

Parthenogenesis (Greek: parthenos = virgin, genesis = origin) = development of an organism from an unfertilised egg without fertilisation by a sperm. Types: Arrhenotoky: unfertilised eggs → males (as in honeybees). Most common form. Thelytoky: unfertilised eggs → females (as in some ant and bee species). Deuterotoky: unfertilised eggs → both males and females. Natural parthenogenesis occurs in: Honeybees (arrhenotokous), Ants, Wasps (Hymenoptera), Aphids (seasonal parthenogenesis — parthenogenesis in summer, sexual reproduction in autumn), Komodo dragons, some sharks and rays (facultative parthenogenesis when no male available), Whiptail lizards (obligate parthenogenesis — all female species). Artificial parthenogenesis: inducing development in unfertilised eggs using chemical or physical stimuli. Used in research and in some forms of assisted reproduction. Historically used to study early embryonic development.

The haplodiploid system of Hymenoptera creates unusual genetic relatedness values that help explain the evolution of worker sterility (altruism). In diploid organisms: full sisters share 50% of genes (r = 0.5). In haplodiploid Hymenoptera: because the drone father is haploid → all his sperm are genetically identical → all daughters of the same mother AND father share 75% of genes (r = 0.75)! Hamilton's rule: altruistic behaviour evolves when rb > c, where r = relatedness, b = benefit to recipient, c = cost to altruist. With r = 0.75 (worker-to-sister), the genetic benefit of helping rear sisters exceeds the cost of sterility more easily than in diploid organisms (r=0.5). This is one explanation for why eusociality (sterile worker castes) has evolved multiple times in Hymenoptera. Workers pass on their genes more effectively by helping raise sisters (75% related) than by reproducing directly (50% related to own offspring).

Caste determination in honeybees is epigenetic — same genome (diploid female), different developmental outcome based on nutrition. Royal jelly: protein-rich secretion from worker bee hypopharyngeal and mandibular glands. Contains: royalactin (key protein that activates EGFR signalling → triggers queen development), 10-hydroxy-2-decenoic acid (10-HDA), royamines (phenylbutyrate derivatives). Queen cell feeding: larvae reared as queens receive royal jelly continuously and in larger amounts throughout larval development. Worker cell feeding: larvae receive royal jelly for first 3 days then switch to bee bread (pollen+honey). Epigenetic mechanism: DNA methylation differences between queen and worker — feeding affects methylation patterns → different gene expression → different developmental programmes from identical genotypes. This is a remarkable example of how nutrition/epigenetics can override genotype in development.

Colony Collapse Disorder (CCD): dramatic die-off of honeybee colonies where worker bees disappear, leaving queen and honey but no adult workers. First reported in USA in 2006-07, now global phenomenon. Causes (multiple, synergistic): Varroa destructor mite: parasitises honeybee brood, vectors multiple viruses (Deformed Wing Virus, etc.). Neonicotinoid pesticides: systemic insecticides in pollen and nectar → impair navigation, learning, immune function. Nosema fungi (N. apis, N. ceranae): gut parasites that weaken bee immunity. Habitat loss: reduced wildflower availability → nutritional stress. Climate change: altered flowering times, extreme weather. Monoculture agriculture: nutritional diversity reduced. Importance: ~75% of food crops depend at least partially on animal pollination. Bees pollinate vegetables, fruits, nuts, oilseeds — contributing to food security. Annual value of bee pollination services: $260-570 billion globally. Conservation measures: reducing pesticide use, planting bee-friendly wildflowers, maintaining hedgerows, supporting organic farming, controlling Varroa.