Genetic regulation of mammalian diversity

Abstract

Mammals have evolved a variety of morphological adaptations that have allowed them to compete in their natural environments. The developmental genetic basis of this morphological diversity remains largely unknown. Bats are mammals that have the unique ability of powered flight. We have examined the molecular embryology of bats and investigated the developmental genetic basis for their highly derived limbs used for flight. Initially, we developed an embryo staging system for a model chiropteran, Carollia perspicillata, the short-tailed fruit bat that has subsequently been used for staging other bat species. Expression studies focusing on genes that regulate limb development indicate that there are similarities and differences between bats and mice. To determine the consequences of these expression differences, we have conducted an enhancer switch assay by gene targeting in mouse embryonic stem cells to create mice whose genes are regulated by bat sequences. Our studies indicate that cis-regulatory elements contribute to the morphological differences that have evolved among mammalian species.

FIGURE 1

Limb comparisons between mouse and bat. Diagram of the limb skeletal elements of the mouse and bat.

FIGURE 2

Limb development in the bat and mouse. Whole-mount images of forelimbs from bat (A–F) and mouse (G–L). Carollia stages 14 (A), 15E (B), 15 (C), 16 (D), 16L (E), 19 (F); mouse stages 10 dpc (G), 11 dpc (H), 12 dpc (I), 13 dpc (J), 14 dpc (K), 16 dpc (L). Anterior is to the right. (Reprinted, with permission, from Cretekos et al. 2008.)

FIGURE 3

Development of the cartilaginous skeleton of Carollia perspicillatta. Whole-mount alcian blue staining of bat embryos from CS 14 through CS 20. The first column shows lateral views with anterior to the top and dorsal to the left. The second column shows high-magnification views of dissected forelimbs shown with anterior to the top and proximal to the left. The third column shows high-magnification views of dissected hindlimbs shown with anterior to the top and proximal to the left. Embryos are staged according to Cretekos et al. (2005) and indicated for each row. Views are not to scale.

FIGURE 4

Bat-specific characters develop late in gestation. Carollia forelimb skeletons at CS 17 (A) and newborn (B). (A) Alcian blue preparation, (B) alcian blue/alizarin red preparation. Digits are numbered I–V. (dpc) Days postcoitus, (H) humerus, (M) metacarpel, (P) phalange, (R) radius, (U) ulna. (B, Reprinted, with permission, from Chen et al. 2005 [© Wiley].)

FIGURE 5

Prx1 expression and mutant forelimb phenotype in the mouse. (A) Prx1 whole-mount in situ hybridization of 11.5-dpc mouse embryo. Expression is detected predominantly in the craniofacial region and limbs. (B,C) Alcian blue/alizarin red forelimb skeleton preparations from wild-type (B) and Prx1^−/−(C) newborn mice.