In the vast realm of insects, certain beetles, mites, and other insects—including bees, ants, and wasps—use a unique system of sex determination known as haplodiploidy. In this system, the sexual destiny of each individual is determined at the time the egg is laid and hinges on a simple but crucial event: fertilization.
Bees, ants, and wasps, which are part of the order Hymenoptera (hymenopterans), share this biological feature that might seem unusual to us: males develop from unfertilized eggs, while females emerge from those that have been fertilized. This process raises fascinating questions about the evolution, biology, and social organization of these species and has been proposed as one of the reasons that explains the complex social life and altruism that characterize these insects.
In the haplodiploid system, males are haploid (having only one copy of each chromosome), while females are diploid (possessing two copies of each chromosome). This chromosomal difference is established when the female lays the egg: if the egg is fertilized by sperm, the resulting offspring will be female with two sets of chromosomes; if left unfertilized, it will become a haploid male, with just one chromosome set identical to the genetic material received from its mother.
This system is fundamental to hymenopterans and other insect groups. In these orders, sex determination does not depend on the presence of sex chromosomes, as in the human XY system, but instead is based on ploidy, or the number of chromosome sets an organism possesses. Diploid individuals will be female, while haploid individuals will develop as males.

In cases of extreme inbreeding, such as when siblings mate with each other, diploid males may appear, though they do not survive since nurse bees in colonies often eliminate them.
One intriguing aspect of the haplodiploid system is the female’s ability to control the sex of her offspring. Queens of species like honeybees store sperm in an internal structure called the spermatheca. When a fertilized female lays an egg, she can choose whether to release sperm to fertilize it, thereby controlling whether the egg will develop into a diploid female or a haploid male. This mechanism enables females to adjust the sex ratio according to the needs of the colony, optimizing resources and reproductive efforts.
The haplodiploid system has profound consequences for kinship, especially in species like bees, ants, and certain wasps. In these species, the genetic structure of kinship among individuals changes fundamentally due to the haplodiploid nature of males.
In a scenario where the queen has mated only once with a male, the female worker bees (who share the same mother and father) on average share three-quarters of their genes with each other. In other words, sisters share more genes with each other than with their mother or daughters. This 3/4 relatedness coefficient is higher than that typically shared between siblings in species with conventional sexual reproduction, where sibling relatedness is about 50%.
This high level of relatedness between sisters has led many scientists to propose the theory of kin selection as an explanation for the emergence of eusociality, a complex social behavior in which individuals cooperate to rear the colony’s young rather than produce their own individual offspring.

According to this theory, the behavior of the worker bees, who dedicate their lives to caring for the queen’s eggs rather than reproducing themselves, can be explained by the fact that by helping their sisters survive, they are transmitting a higher proportion of their own genes. Since the workers are more closely related to their sisters than to potential offspring of their own, caring for and protecting the queen’s offspring maximizes the genetic success of their own genes more than if they produced their own offspring.
This, however, becomes more complex in colonies where queens mate with multiple males, as occurs in many bee species. In these cases, not all workers will be equally related to each other, potentially generating conflicts of interest in rearing strategies. In the case of bees, only 10 percent of the species are social.
In some cases, females lay unfertilized eggs, which will become males, in lower-quality food sources, while fertilized eggs, destined to become females, are laid in higher-quality food sources. This strategy could be because the reproductive success of females depends more on resource availability than that of males.
This pattern is observable in other haplodiploid species, such as ambrosia beetles, where a higher number of males is found in populations with greater opportunities for dispersal and mating. Manipulating the sex ratio based on environmental conditions seems to be an adaptive strategy that maximizes reproductive efficiency, adjusting the sex ratio according to the availability of resources and mating opportunities.
Haplodiploidy in bees, ants, and wasps represents a fascinating evolutionary adaptation that, far from being a mere reproductive mechanism, has shaped complex social structures and cooperative behaviors that have intrigued biologists for decades. This system of sex determination allows for significant flexibility in managing offspring, fosters altruism, and enables the emergence of eusocial behaviors, all within an evolutionary logic that maximizes the genetic success of individuals through cooperation and teamwork.
This article was first published on our Spanish Edition on November 20, 2024: En abejas, hormigas y avispas los machos nacen de huevos no fecundados y las hembras de los que sí lo han sido, y es la reina quien lo decide
SOURCES
E.O. Wilson, B. Hölldobler, Eusociality: Origin and consequences, Proc. Natl. Acad. Sci. U.S.A. 102 (38) 13367-13371, doi.org/10.1073/pnas.0505858102
Michael Mahowald, Eric von Wettberg, Sex determination in the Hymenoptera
David Grimaldi, Michael S. Engel, Evolution of the Insects
Wikipedia, Haplodiploidía
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