Evolutionary and Phenotypic Plasticity of Mammalian Kidney: Using the Laboratory House Mouse as a Model
Under the supervision of Professor Theodore Garland, Jr.
Department of Zoology
At the University of Wisconsin-Madison
Relative medullary thickness (RMT) is the most commonly
used morphological indicator of urine concentrating ability of mammalian
kidneys. I tested the assumed positive relationship of RMT to habitat
aridity in rodents, using both conventional and phylogenetically informed
analysis of covariance (ANCOVA with body mass as covariate). Body
mass, mass-corrected kidney mass, mass-corrected RMT, mass-corrected maximum
urine concentration, and habitat (scored on a scale of 1 to 4 to indicate
increasing aridity) all showed significant phylogenetic signal. Mass-corrected
RMT and maximum urine concentration showed a highly significant positive
correlation (N = 38 species). Mass-corrected RMT varied significantly
among habitats (N = 144), and a phylogenetic (but not a conventional) analysis
indicated that body mass-corrected kidney mass (N = 104) was positively
related to habitat.
I next examined the effect of early-age water restriction on the developmental plasticity of male outbred laboratory house mice (Hsd:ICR). Beginning at weaning, one group was raised with ad lib water (FW-J), while another experienced gradual water-restriction (WR-J) for two months. Water restriction reduced food consumption, slowed growth, and changed relative kidney, adrenal, heart, epididymides, and testes size. Kidney changes included hypertrophy of both outer and inner medullas, which resulted in increased RMT. WR-J voided more concentrated urine, and their renal tubules had more vasopressin receptors. However, the magnitude of differences in RMT and in kidney mass induced by water restriction were substantially less than has been reported for differences among rodent species.
To investigate reversibility of plasticity, four other groups were studied: FW had ad lib water for two additional months, WR had continued water restriction, FWWR had ad lib then restricted water, and WRFW experienced restriction followed by ad lib water. Many of the changes observed in the previous experiment were largely reversed when the water regimen was switched. For example, mice of the WRFW group grew rapidly after the switch to free water, but never caught up to the FW group. Mice of the FWWR group shrank in both mass and length when water restricted, but always remained larger than the WR group. Interestingly, I found no group differences in RMT.