When determining sex, exceptions are the rule
For nearly a century, biologists have modeled the evolution of sex chromosomes—the genetic instructions that primarily determine whether an individual will develop into a male or female (or a certain mating type)—resulting in an impressive theoretical framework. Now, thanks to the publication of genomic data from a wide variety of non-model organisms, these theories are being tested against empirical evidence from nature—often with surprising results. In a new review in Genome Biology and Evolution, Benjamin Furman, Judith Mank, and colleagues detail the surprising number of exceptions to the purported rules of sex chromosome evolution theory, revealing the potential limitations to our understanding of chromosome-based sex determination systems.