Autosome

One-sentence definition. An autosome is any chromosome that is present in the same number and form in both sexes — every chromosome except the sex chromosomes.

One-sentence analogy. If you imagine a car where most parts are identical between two models, autosomes are all the shared parts; the sex chromosomes are the optional add-on that differs between the two versions.

Why it matters. Sex chromosomes evolve from ordinary autosomes through a process driven by sexual antagonism and recombination suppression. Understanding which chromosomes are autosomes — and how many there are — is fundamental to interpreting karyotypes, modeling chromosome number evolution, and identifying the precursors of neo-sex chromosomes. The X chromosomes of Drosophila melanogaster and Anopheles gambiae are each homologous to unique autosomes in the other species, illustrating that sex chromosomes have independent autosomal origins.

Where you meet it in the wiki.

• Sex chromosome evolution — sex chromosomes originate from autosomes.
• Chromosome fusion — sex chromosome–autosome fusions create neo-sex chromosomes.

Primary citation.

“The shared region of the D. melanogaster and Anopheles gambiae X is not homologous to the X of the beetle Tribolium castaneum or the Z of the silk moth Bombyx mori, but in each case the X (or Z) is homologous to a unique autosome in the other species.” — Blackmon & Demuth 2015, Finding 3

Prerequisites: none Next, learn about: heterogamety, sex determination, chromosome fusion

Background

The term autosome traces to the cytologist Edmund B. Wilson, who in the early twentieth century was sorting out the chromosomal basis of sex determination. Wilson needed a name for the chromosomes that behaved identically in both sexes, as distinct from the heteromorphic pair he was tracking. He introduced autosome to carry that specific meaning: a chromosome present in matched copies in both sexes, contributing equally to both sons and daughters. The term stuck because it solved a real classification problem, and it remains the standard today.

For most of the twentieth century, autosomes were treated as the quiet background of the karyotype. Sex chromosomes attracted attention because they differ between sexes, evolve rapidly, and carry genes relevant to sexual development. Autosomes were treated as stable, interchangeable, and selectively neutral at the chromosomal level. That framing has changed substantially. We now know that autosome number, size, and fusion state vary substantially across lineages, and that the rate of autosomal rearrangement is shaped by effective population size, the meiotic machinery, and the presence or absence of sexual antagonism.

The conceptual pivot that connects autosomes to sex chromosome biology is the recognition that sex chromosomes are not a permanent category. Every sex chromosome was once an autosome. Two homologous autosomes acquire a sex-determining locus, recombination between them begins to break down, and a pair of sex chromosomes assembles from what was ordinary autosomal material. This origin story, now supported by comparative genomic data across insects, reptiles, fish, and plants, means that understanding autosomes is a prerequisite for understanding sex chromosome evolution.

How it works

An autosome behaves symmetrically across sexes during meiosis. In a diploid organism with 2n chromosomes, autosomes contribute equally to eggs and sperm; any given autosome has the same probability of being transmitted through either sex. This symmetry is what distinguishes autosomes from sex chromosomes, which have biased transmission (e.g., the Y chromosome passes only through fathers). Because autosomes are transmitted symmetrically, alleles on autosomes experience the same selective environment in males and females, averaged across generations.

Autosome number is not fixed within lineages. Fissions and fusions change the number of autosomes, and these rearrangements can fix or be lost depending on their fitness effects and the effective population size of the lineage. A Robertsonian translocation, for example, fuses two acrocentric autosomes at their centromeres to produce a single metacentric chromosome, reducing autosome count by one. Dysploidy, the gradual stepwise change in chromosome number, operates through exactly these kinds of autosomal rearrangements. The meiotic machinery matters here: in species with achiasmy (no crossing over in one sex), autosomal rearrangements face different segregation constraints than in species where both sexes cross over.

The fusion of an autosome to a sex chromosome creates a neo-sex chromosome. When this happens, the autosomal segment carried by the new neo-X or neo-Y begins to evolve under the same forces that shaped the original sex chromosomes: recombination suppression near the sex-determining region can spread, sexually antagonistic alleles on the autosomal segment experience selection through only one sex, and the dosage of genes on that segment changes between males and females. A formerly ordinary autosome enters a qualitatively different evolutionary trajectory. These sex-autosome fusions are a key mechanism by which sex chromosomes grow over time.

A worked example

In beetles of the family Tenebrionidae, the flour beetle Tribolium castaneum has a karyotype of 2n = 20, with nine pairs of autosomes and one pair of sex chromosomes (XX/XY). The nine autosome pairs are present in equal copy number in both sexes and segregate normally. Work in the Blackmon lab using comparative karyotype data across Coleoptera finds that autosome number varies from as few as 2 to more than 40 across beetles, and that lineages with low effective population size (e.g., those with low dispersal or boom-bust dynamics) show elevated rates of autosomal rearrangement. This pattern is consistent with rearrangements that are slightly deleterious fixing by drift rather than selection, and it illustrates that autosomes are not a static backdrop but an evolving part of the genome architecture.

Common misconceptions

• Misconception: autosomes never carry sex-biased genes. Autosomes carry many genes with sex-biased expression; what they lack is sex-biased transmission, not sex-biased function.
• Misconception: the number of autosomes is fixed within a species. Chromosomal rearrangements, including fusions and fissions, can change autosome number and occasionally segregate as polymorphisms within populations.
• Misconception: autosomes are immune to sexual antagonism. Sexually antagonistic alleles can occur on autosomes; however, without linkage to a sex-determining locus, they cannot be sexually restricted in the same way as alleles on sex chromosomes.
• Misconception: sex chromosomes are categorically different from autosomes in origin. Sex chromosomes originate from autosomes and can revert to autosome-like behavior if the sex-determining locus is lost or translocated.
• Misconception: autosome count equals the haploid chromosome number minus one. The haploid chromosome number includes sex chromosomes; the autosome count is the total minus the number of sex chromosomes, which may be more than one (e.g., X1X2Y systems).

How to spot it in papers

• Karyotype figures label chromosomes with numbers (1, 2, 3…) for autosomes and letters (X, Y, Z, W) for sex chromosomes; any numbered chromosome is an autosome.
• Phylogenetic comparative studies of chromosome number typically report autosome number separately from sex chromosome number and model them with separate rate parameters.
• Papers on neo-sex chromosomes explicitly name which autosome fused to the ancestral sex chromosome, often by synteny analysis or homology mapping to an outgroup.
• Tables reporting karyotype data often have a column labeled “2n” and a separate column labeled “sex chromosome system”; the autosome count is 2n minus the sex chromosome count.
• Papers using BiSSE or related state-dependent diversification models on karyotype data encode autosome number as the continuous or discrete trait, distinct from sex chromosome state.

Further reading

• Sex determination — how autosomes acquire sex-determining loci and begin the transition to sex chromosomes.
• Neo-sex chromosome — the evolutionary trajectory of autosomal segments after fusion to a sex chromosome.
• Robertsonian translocation — the most common mechanism by which autosome number decreases through centromeric fusion.