Developmental basis of mammalian digit reduction: A case study in pigs

Digit reduction has occurred in parallel in many mammalian lineages. However, despite this pattern's prevalence, the developmental mechanisms underlying mam- malian digit reduction remain controversial. We therefore un- dertook a study of digit development in the pig (Sus scrofa), a mammal with reduced first, second, and fifth digits. Our re- sults indicate that from its earliest formation, the pig limb bud is significantly narrower than that of the model pentadactyl mammal, mouse. Furthermore, the cartilage condensations of the pig's reduced digits are noticeably smaller than those of their nonreduced counterparts from the time of their for- mation. In addition, growth rates of pig digits are comparable, as are the patterns of cell death in developing pig and mouse limbs. Taken together, results suggest that pig's first, second, and fifth digits are primarily reduced through evolu- tionary modifications in the early developmental patterning of their limbs. Results of this study, coupled with those from study of limb development in other mammals, suggest that al- though major developmental reorganizations (e.g., complete digit or limb loss) during early limb development may be se- lected against, it may be common for more subtle evolutionary modifications in limb development (e.g., changes in relative digit size) to occur at this time.