Developmental expression of the mouse Evx-2 gene: relationship with the evolution of the HOM/Hox complex.

The mouse Evx-2 gene is located in the immediate vicinity of the Hoxd-13 gene, the most posteriorly expressed gene of the HOXD complex. While the Evx-1 gene is also physically linked to the HOXA complex, it is more distantly located from the corresponding Hoxa-13 gene. We have analysed the expression of Evx-2 during development and compared it to that of Evx-1 and Hoxd-13. We show that, even though Evx-2 is expressed in the developing CNS in a pattern resembling that of other Evx-related genes, the overall expression profile is similar to that of the neighbouring limbs and genitalia. We propose that the acquisition of expression features typical of Hox genes, together with the disappearance of some expression traits common to Evx genes, is due to the close physical linkage of Evx-2 to the HOXD complex, which results in Evx-2 expression being partly controlled by mechanisms acting in the HOX complex. This transposition of the Evx-2 gene next to the Hoxd-13 gene may have occurred soon after the large scale duplications of the HOX complexes. A scheme is proposed to account for the functional evolution of eve-related genes in the context of their linkage to the HOM/Hox complexes.