How Many Chromosomes Does a Butterfly Have?

Chromosomes are the tightly packaged carriers of genetic information within the cells of every living organism. Composed of DNA and proteins, these structures contain the blueprint that dictates an organism’s traits. The number of chromosomes is a characteristic of a species, and this number can vary significantly across the diversity of life, offering a window into evolutionary history.

Butterfly Chromosome Numbers: A Surprising Variety

There is no single answer to how many chromosomes a butterfly has, as the number is highly diverse across the order Lepidoptera, which includes both butterflies and moths. This group of insects exhibits one of the greatest ranges in chromosome number in the animal kingdom. The count is expressed by a haploid number (n), representing a single set of chromosomes in reproductive cells. The total number in most body cells, the diploid number (2n), is double the haploid count.

The known haploid numbers in butterflies span from as low as n=5 to as high as n=223. While most species have a haploid number around n=29 to n=31, the likely ancestral state, some families display extreme deviations. For instance, the Monarch butterfly (Danaus plexippus) has a haploid number of n=30. In contrast, some species in the genus Polyommatus, like the Atlas blue butterfly (Polyommatus atlantica), possess a high count of up to 229 chromosomes.

This karyotypic diversity is concentrated in families like the Lycaenidae (blues, coppers, and hairstreaks) and Nymphalidae (brush-footed butterflies). The variability within these groups highlights that chromosome number is not a static trait. It is a dynamic feature that has changed over the course of butterfly evolution.

Factors Influencing Chromosome Variation in Butterflies

The wide range in chromosome numbers among butterflies results from evolutionary processes that rearrange the genome. The primary mechanisms are chromosome fission and fusion. Fission occurs when a single chromosome breaks into two smaller ones, while fusion is when two separate chromosomes join to create one larger chromosome.

The structure of butterfly chromosomes, which are holocentric, may facilitate these changes. Unlike monocentric chromosomes in humans that have a single centromere, holocentric chromosomes have this function distributed along their length. This difference may make fragments from fission more likely to remain viable, allowing for faster evolution of the karyotype.

These chromosomal rearrangements can lead to the formation of new species, a process called speciation. When a population’s chromosome structure changes significantly, it can become reproductively isolated from its ancestral population. Individuals from the two groups may no longer produce viable offspring, creating a barrier that allows them to diverge into distinct species.

How Butterfly Chromosomes Compare

The range of chromosome numbers in butterflies is notable when compared to other organisms. Humans have a diploid number of 2n=46 (n=23). The common fruit fly, Drosophila melanogaster, has one of the lowest numbers among insects, with a diploid count of 2n=8 (n=4).

In contrast, the butterfly order Lepidoptera has a haploid range from n=5 to over n=220. Some butterfly species have fewer chromosomes than a fruit fly, while others, like the Atlas blue (Polyommatus atlantica), have a count that far exceeds that of humans. This spectrum illustrates there is no simple correlation between an organism’s complexity and its chromosome number.

What Chromosomes Reveal About Butterflies

Studying chromosome number and structure, a field called karyotyping, provides insights into butterfly biology. One practical application is in species identification. For “cryptic species,” where distinct species are nearly identical in appearance, differences in chromosome counts can confirm their separation.

Chromosomes also determine an individual’s sex. Butterflies use a ZW sex-determination system, unlike the XY system in humans. In the ZW system, males are ZZ, while females are ZW. This makes the female the heterogametic sex, as her reproductive cells determine the offspring’s sex.