Different genes, same result: How flies evolved multiple ways to define heads and tails

The process of developing an organism’s head and tail in early life is a crucial milestone. Scientists have studied this phenomenon extensively using the fruit fly (Drosophila melanogaster) as a model organism. A gene called bicoid is responsible for establishing the anterior-posterior body axis in fruit flies. However, researchers have discovered that other fly species have evolved alternative ways to achieve the same result.

In 2019, Dr. Urs Schmidt-Ott from the University of Chicago and his team identified three unrelated genes that perform the same function in other fly species. This phenomenon, known as developmental system drift, refers to the divergence of developmental gene networks while preserving the outcome, which in this case is the head-to-tail axis.

A recent study published in PLOS Biology focuses on how a gene called odd-paired establishes the head-to-tail axis in a moth fly, Clogmia albipunctata. Similar to bicoid in fruit flies, odd-paired alters chromatin accessibility, a measure of how open or closed a region of DNA is inside the cell nucleus. Open chromatin allows genes to be expressed and regulated more easily, while closed chromatin restricts access to genetic machinery.

In moth fly embryos, the expression of the odd-paired gene ensures the correct number of body segments form as the larval body plan develops. Interestingly, maternal expression of odd-paired occurs earlier, during egg formation, providing a nearly identical protein at a different time and location in the prospective head region of the egg. This localized early activity of odd-paired allows it to be reused to break axial symmetry in moth flies.

The researchers also investigated how odd-paired functions at the molecular level and identified its downstream targets, which differ from those of bicoid in fruit flies. While bicoid has dozens of direct target genes, the odd-paired substitute in moth flies may not target hunchback, a well-studied early target gene in fruit flies. Instead, the moth fly's odd-paired substitute activates two different genes, homeobrain and sloppy-paired, to initiate head development.

The study highlights the importance of studying multiple species to understand how nature solves the same problem in different lineages. With over 150,000 described fly species, there is significant variation in how they establish head-to-tail polarity in the embryo. By comparing the developmental gene networks of various fly species, researchers can gain insights into the principles governing the evolution of these mechanisms.

This comparative approach can help answer questions about which features of genetic networks are most resistant to change and why.