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Study reveals the genetic basis of butterfly sex

In the world of biological research, understanding the mechanisms that determine sex is fundamental. While genes of humans and other mammals rely on the familiar X and Y chromosomes, nature displays a remarkable diversity of solutions to this essential process. A new study sheds light on the genetic basis of sex determination in a butterfly – and reveals potential threats to endangered populations.

Butterfly sex determination

Butterflies, as well as moths, belong to the insect order Lepidoptera. Unlike humans who rely on the X and Y chromosome system for sex determination, Lepidoptera utilize a WZ system.

In this system, females possess two different sex chromosomes, a W and a Z, while males have two Z chromosomes. However, the story doesn’t end there. The way this system translates into male or female development differs significantly between butterfly species.

In some butterfly species, the presence of the W chromosome is the key factor that triggers development into a female. Conversely, other species lack a W chromosome in females altogether.

This remarkable diversity in sex determination mechanisms within Lepidoptera has puzzled researchers for quite some time. They’ve been working to gain a more comprehensive understanding of how butterflies develop into males or females across this vast insect group.

Squinting bush brown butterfly

To delve into the mysteries of butterfly sex determination, researchers from the Biology Centre of the Czech Academy of Sciences and the University of Liverpool turned their attention to the squinting bush brown butterfly (Bicyclus anynana). This African species has become a favored organism for research for several reasons:

  • Genetic roadmap: Scientists have fully sequenced the squinting bush brown’s genome, providing a detailed map for investigation.
  • Laboratory-friendly: The butterfly thrives in controlled laboratory settings, readily reproducing and offering researchers ample material to study.

The research team’s investigation yielded a vital discovery. They identified a single gene, which they named Masculinizer, as the primary factor controlling sex determination in B. anynana. The specific variations of this gene, known as alleles, determine whether an individual butterfly develops as a male or a female.

One allele of Masculinizer triggers female development. When a butterfly inherits two different alleles of the gene, male development ensues. However, embryos possessing two identical Masculinizer alleles do not survive. This lethal combination underscores the importance of genetic diversity: in large populations with a rich variety of Masculinizer alleles, encounters between identical versions are rare.

“These species have different sex-determining mechanisms which are not known yet. The mechanism we discovered in Bicyclus anynana is very different from the W chromosome-dependent mechanism discovered earlier in the silkworm Bombyx mori,” explained the researchers.

Convergent evolution with honeybees

The study on the squinting bush brown butterfly revealed a surprising twist. The sex determination system identified in this butterfly closely resembles the mechanism at play in honeybees. This remarkable similarity is a prime example of convergent evolution, which is when unrelated organisms develop strikingly similar traits due to facing similar environmental pressures.

In this case, both butterflies and honeybees, despite belonging to distinct insect groups, appear to have arrived at the same solution for sex determination through independent evolutionary pathways.

This shared sex determination system suggests the possible existence of fundamental genetic pathways that govern sex differentiation across a wider range of insects. The discovery in B. anynana hints at a deeper evolutionary connection, where these pathways might have originated from a common ancestor and then diverged over time as these insect groups evolved separately.

Butterfly sex and conservation

The discovery has significant implications for butterfly conservation. Many butterfly species are strongly in decline. Population declines lead to reduced genetic diversity.

This, in turn, increases the likelihood of embryos inheriting identical, incompatible Masculinizer alleles. The potential loss of offspring could exacerbate the challenges faced by dwindling butterfly populations.

This study highlights the value of fundamental research into seemingly simple biological questions. Understanding the mechanics of sex determination in an unassuming butterfly species has uncovered hidden risks to vulnerable populations – a testament to the far-reaching impact of seemingly small genetic variations.

The study is published in the journal Science Advances.


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