Kathleen Smith's Lab

Anna Keyte, Ph.D. email address: alk2

My primary research interest is the evolution of development and how changes in developmental programs have contributed to the species diversity we see today. My dissertation research has focused on the role of heterochrony (a change in developmental timing) in the morphological evolution of mammals. I have used early limb development and somitogenesis (segmentation) in the Brazilian short- tailed opossum, Monodelphis domestica, as a model to study the genetic and developmental origins of heterochrony.

 

 

 

Opossum and other marsupial neonates are born in an altricial state compared to eutherian neonates, such as mouse. Gestation is relatively short while maternal investment emphasizes a longer period of lactation. The marsupial neonate has an overall embryonic state of development with certain features accelerated relative to eutherian embryos. For instance, the forelimbs of newborn opossums are well enough developed to allow them to crawl unaided from the opening of the birth canal to the teat. In contrast, the hind limbs are strikingly smaller, non-functional, and relatively under-developed. In marsupials that have a particularly short gestation period and are super-altricial at birth, this difference in the limbs is even greater. Changes in the timing of somitogenesis have also occurred, presumably to allow precocial development of anterior somitic derivatives required at birth. My dissertation research aims to uncover the developmental and genetic bases of limb and somitogenic heterochrony in these neonates.

Much research into vertebrate segmentation assumes that somites form at a constant rate. Very little work has investigated changes in rate during embryogenesis. I have quantified the dynamics of somitogenesis rate in M. domestica by counting the number ofsomites in embryos of various ages. When compared to mouse, the most anterior somites in the opossum form very quickly while there is a dramatic slowing of somitogenesis posteriorly. As a follow up, I have examined expression patterns of several genes known to be involved in the somite clock in model systems. The function of these particular genes appears to be conserved in the opossum. I have also adapted a computer model from Lewis (2003) to model clock slowing in an embryo over developmental time. My ultimate goal is to determine what inputs to the somite clock have been modified in marsupials to allow the change in rate.

When the hind limbs of opossum embryos begin outgrowth, the forelimbs are already well-defined buds. In eutherian mammals and other vertebrates in general, the limb buds are similar sizes at this stage of development, indicating that limb heterochrony arises very earl  in the opossum. With the assistance of a talented undergraduate in the lab (Beena Imam), I have looked at gene expression upstream of forelimb and hind limb outgrowth to pinpoint the genetic change(s) that may have contributed to limb heterochrony. As a proxy for limb field specification, we have examined the expression of genes used previously as limb field markers (Tbx5 for the forelimb and Tbx4 for the hind limb), as well as downstream genes known to be involved in outgrowth of both limbs (Fgf10 and Fgf8). Using whole mount in situ hybridization and opossum-specific probes, we have compared the timing of earliest limb expression of each of these genes in M. domestica and Mus musculus (mouse). We have found that the timing of first Tbx5 expression in the future forelimb region has shifted much earlier in the opossum, as has the expression of Fgf10 and Fgf8. Tbx4 is also expressed earlier in the opossum, but appears so early relative to caudal axis development that hind limb outgrowth may be limited by tissue availability. In addition, a subset of the Hox genes has been hypothesized to lie upstream of Tbx5/4 in positioning the limbs. We are in the process of testing this by examining the timing and pattern of relevant Hox expression in M. domestica.

 

Anna defended her dissertation in January 2010 and is now a member of Peggy Kirby's lab

 

Publications:

Keyte, A. L., R. Percifield, B. Liu and J. F. Wendel. 2006.   Infraspecific DNA
Methylation Polymorphism in Cotton (Gossypium  hirsutum L.).  Journal of Heredity 97(5): 444-450.

Keyte, A. L., T. Imam and K. K. Smith. 2006.  Limb heterochrony in a  marsupial, M. domestica. Developmental Biology 295: 249. (abstract)

 


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updated April 2007