Ants, members of the family Formicidae, are a successful and complex group whose intricate social structures are governed by a unique genetic architecture. Chromosomes are filamentous structures found within the nucleus of every cell, serving as the carriers for an organism’s genetic blueprint, or DNA. Examining the number and structure of these chromosomes provides insights into the genetic basis for the diversity seen in the ant family.
The Typical Chromosome Count
The number of chromosomes an ant has depends on both the species and the ant’s sex. For many well-studied species, such as those in the genus Lasius (common garden ants), the haploid number (n) is typically 15. This refers to a single set of chromosomes, which males possess, meaning they have 15 chromosomes in their body cells.
Females of the same species, including queens and workers, are diploid (2n), possessing two complete sets of chromosomes, one inherited from each parent. This diploid number is exactly double the male’s count, resulting in 30 chromosomes for Lasius females. This pattern of a doubled chromosome count in females compared to males is a defining feature of the entire order Hymenoptera (ants, bees, and wasps). Other common groups, such as the genus Formica, have a higher haploid number, often 26 or 27, resulting in 52 or 54 chromosomes for the female caste.
Chromosomes and Sex Determination
The difference in chromosome number between the sexes is the direct result of a specialized reproductive system known as haplodiploidy. This mechanism is the primary way sex is determined in ants and is central to their social organization. The fertilization status of an egg dictates the sex of the developing ant.
Females (queens and workers) develop from eggs that have been fertilized by a male sperm cell. The fusion of the egg’s haploid set with the sperm’s haploid set creates a diploid (2n) individual. These diploid individuals are always female, though their caste is determined by environmental factors like nutrition during development.
In contrast, males, commonly referred to as drones, develop from unfertilized eggs. The unfertilized egg contains only the mother’s single, haploid set of chromosomes, and the egg begins to develop without any genetic contribution from a father. This results in the male having a haploid (n) number of chromosomes in every cell of its body.
The mechanism is driven by a single, complementary sex-determination (CSD) locus. Heterozygosity (having two different versions of the gene) at this locus results in a diploid female. If an egg is fertilized by a male that shares the same CSD allele as the queen, the offspring will be homozygous, resulting in a diploid male. These diploid males are usually sterile, which acts as a genetic cost to inbreeding within the colony.
Diversity Across Ant Species
While many ant species share a conserved chromosome count, the Formicidae family exhibits one of the widest ranges found in the animal kingdom. The haploid number across different species ranges dramatically from as low as n=1 to as high as n=53. This variation highlights the dynamic nature of ant karyotype evolution.
On the low end is a sibling species of the Australian jack jumper ant, Myrmecia pilosula, whose males have a haploid number of n=1 (a single chromosome). The females of this species are diploid (2n=2), possessing only two chromosomes. This represents the lowest possible chromosome number in any eukaryotic organism.
At the opposite extreme, the Neotropical giant ant Dinoponera lucida holds the record for the highest count, with a haploid number of n=53 (106 diploid chromosomes). Another example is the primitive ant Nothomyrmecia macrops, which has a diploid count of 94 (2n=94). This massive fluctuation is believed to be the result of evolutionary processes like Robertsonian translocations, where chromosomes fuse or split.