Honeybees (Apis mellifera) exemplify haplodiploidy — one of the most fascinating sex determination systems. Females (queens and workers): diploid (2n = 32). Develop from fertilised eggs (sperm + egg). Queens: reproductive females, fed royal jelly throughout larval development, lay up to 2000 eggs/day. Workers: sterile females, fed royal jelly only for first 3 days then bee bread. Males (drones): haploid (n = 16). Develop from unfertilised eggs by parthenogenesis (arrhenotoky). No father — inherits ONLY the queen mother's genome. Cannot produce gametes by meiosis (already haploid). Instead: haploid spermatocytes divide by mitosis to produce haploid sperm. All sperm from one drone are genetically identical. Sole function: mating with a virgin queen during nuptial flight. Die immediately after mating (genitalia torn away).

Meiosis is specifically designed to halve the chromosome number from 2n to n. Meiosis I (reductional division): HOMOLOGOUS CHROMOSOME pairs align → one of each pair goes to each daughter cell → chromosome number halved (2n → n). For meiosis I to work: chromosomes must come in pairs (homologous pairs). Haploid cells (like drone spermatocytes): have ONLY ONE of each chromosome — no homologous partner. Meiosis I cannot occur (no pairs to separate). Meiosis II: division of sister chromatids (like mitosis). Even if meiosis II could occur in a haploid cell, the result would be cells with half the haploid chromosome number — non-viable. Therefore: haploid cells can ONLY divide by mitosis. Mitosis: each chromatid goes to a daughter cell → same chromosome number (n → n). All resulting cells are haploid and genetically identical (no crossing over).

Parthenogenesis: development from unfertilised egg without fertilisation. Types: Arrhenotoky: unfertilised eggs → males (haploid). Most common. Examples: all Hymenoptera (honeybees, wasps, ants). Thelytoky: unfertilised eggs → females. Examples: some ant species, some bee species, Komodo dragon, some sharks and rays. Deuterotoky: unfertilised eggs → both males and females. Examples: some insects, some crustaceans. Natural examples: Bdelloid rotifers (obligate parthenogenesis — no sexual reproduction). Parthenogenetic lizards (Cnemidophorus, Aspidoscelis in North America — all-female species). Water flea (Daphnia): alternates between sexual and parthenogenetic reproduction. Artificial parthenogenesis: inducing development with chemicals or physical stimuli (heat, pressure). Used in experimental embryology. Some IVF-related techniques. Significance: allows rapid population growth when mates are unavailable, reduces genetic recombination.

Haplodiploidy has evolutionary consequences for kin selection and the evolution of worker sterility. Coefficients of relatedness (r) in honeybees (assuming same mother AND same father): Worker to full sister: r = 0.75 (extraordinary high). Why? Father (drone) has haploid genome → ALL his sperm are genetically identical. Daughters of same drone share 100% of paternal genome (not just 50% as in diploid organisms). Plus 50% maternal genome shared on average. Total: 0.5 x 0.5 + 0.5 x 1.0 = 0.75. Worker to own offspring: r = 0.5 (normal). Since workers are 0.75 related to sisters but only 0.5 to own offspring: it makes evolutionary sense (via Hamilton's rule: rb > c) for workers to be sterile and raise sisters rather than reproduce themselves. This haplodiploidy hypothesis for eusociality was proposed by W.D. Hamilton (1964). Note: eusociality also evolved in termites (diploid) — haplodiploidy not sufficient or necessary, just facilitating.

Different organisms use very different mechanisms to determine sex: XX-XY (humans, Drosophila): sex determined by specific sex chromosomes. Males produce X and Y sperm (normal meiosis). XX-XO (grasshoppers): females XX, males XO. Males produce X and nullo-X sperm (normal meiosis). ZW-ZZ (birds, butterflies): females ZW, males ZZ. Normal meiosis in both sexes. Haplodiploid (honeybees, all Hymenoptera): sex determined by ploidy. Males haploid (produce sperm by mitosis), females diploid (produce eggs by meiosis). Temperature-dependent sex determination (TSD): crocodiles, turtles, some lizards. No sex chromosomes. Temperature during incubation determines sex. Incubation temperature → enzyme activity → hormone levels → gonadal differentiation. For NEET: the key fact about honeybees is that males (drones) are haploid and therefore produce gametes by MITOSIS not meiosis.

In most sexually reproducing diploid organisms: gametogenesis involves meiosis to produce haploid gametes. Male animals (spermatogenesis): Spermatogonia (2n) → mitosis → primary spermatocytes (2n) → meiosis I → secondary spermatocytes (n) → meiosis II → spermatids (n) → spermiogenesis → sperm (n). Female animals (oogenesis): Oogonia (2n) → mitosis → primary oocytes (2n) → meiosis I → secondary oocyte (n) + 1st polar body → meiosis II → ovum (n) + 2nd polar body. Exception — honeybee drones: Spermatocytes (n) → MITOSIS → spermatids (n) → spermiogenesis → sperm (n). This is the only natural example in well-known animals where gametes are produced by mitosis. Key distinction: meiosis in diploid organisms → genetic diversity through crossing over and random assortment. Mitosis in haploid drones → all sperm genetically identical (no variation).

Honeybee colony is a superorganism with ~50,000 workers, one queen, few hundred drones. Communication: waggle dance (discovered by Karl von Frisch, Nobel 1973): a figure-eight dance performed by forager bees on the honeycomb. The angle of the waggle run relative to vertical = direction of food source relative to the sun. Duration of waggle run = distance to food source. Round dance: food source close (<50 m). Pheromone communication: queen mandibular pheromone (QMP) suppresses worker ovary development and worker queen-rearing behaviour. Alarm pheromone (isoamyl acetate): from sting gland → recruits defenders. Nasonov pheromone: orientation and recruitment. Division of labour by age: nurse bees (days 1-12), house bees (days 12-21), guard bees, forager bees (day 21+). Thermoregulation: workers maintain hive temperature at 35-38°C by fanning or clustering. Honey production: foragers collect nectar → pass to house bees → add enzymes → evaporate water → seal with wax.

Haplodiploidy has important genetic consequences: Exposure of recessive alleles: in haploid males, ALL genes are expressed (no masking by dominant allele). Deleterious recessive mutations immediately visible to selection → rapidly eliminated. This may explain why drone populations have been proposed to have cleaner genomes. No sex-linked recessive inheritance pattern: X-linked recessives are more common in diploid male insects (like Drosophila where X-linked recessives are expressed in hemizygous males). In haploid honeybee males: ALL genes expressed regardless of chromosome of origin. Polyandry in queen: queen mates with 10-20 drones during nuptial flight → multiple patrilines in worker force → increased genetic diversity despite haplodiploidy → greater colony resilience. Worker patriline diversity benefits: different patrilines have different disease resistance genes, different temperature preferences → colony better able to respond to varied challenges. This is the proposed adaptive reason for queen polyandry.