Figure 5.

Schematic representation of the main findings and hypotheses generated by this study. (a) E. sp.1 and sp.2 diverged approximately 5 mya and fixed independent fusions involving the same chromosome (represented in blue). The fusion gave rise to a multiple XY sex-determination system in E. sp.2. The E. virescens species pair diverged more recently (<0.6 mya) and one of the species evolved heteromorphic sex chromosomes through the heterochromatinization of the distal region of the X chromosome. A hypothetical inversion might have led to this accumulation of heterochromatin. (b) Possible outcome of a hybridization event involving a E. sp.2 male and a E. sp.1 female. Half of the hybrids would inherit the E. sp.2 Y chromosome. This might lead to complications in the pairing of homologs during the hybrid meiosis. Possible sex ratio distortions and the outcome of the reciprocal cross are harder to deduce given the lack of knowledge of the sex-determination system of E. sp.1. Chromosomes derived from E. sp.1 and 2 are labeled with ^* and + respectively.