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CHAPTER 1: Introduction.
Genetic information from the salamander Ambystoma mexicanum, commonly known as
the Mexican axolotl, provides unique on adaptation and the evolution of vertebrate
genomes. The evolutionary history of A. mexicanum has suited it for these studies in two
important ways. First, the species is a member of the class Amphibia. The amphibian
lineage and the amniote lineage (collectively: mammals, birds, and other reptiles) shared
a common ancestor approximately 350 million years ago. Comparisons between
amphibian and amniote genomes can therefore reveal conserved features that are retained
from the genome of their common ancestor. A second convenient feature of A.
mexicanum is that it is a member of the tiger salamander species complex (sensu Shaffer,
1984a), which is distributed across the southern half of North America and consists of
several closely related species that have adapted to exploit a variety of temporary and
permanent aquatic habitats (Shaffer and McKnight, 1996). Coordinate with their diverse
habitat use, tiger salamanders also exhibit extreme diversity in phenotype (Shaffer,
1984b). Despite this phenotypic diversity, crosses between tiger salamander species
typically result in the production of viable and fertile hybrid offspring (reviewed by Voss
and Shaffer, 1996). When these hybrids are mated, their offspring segregate genotypes
that are inherited from their parental species and phenotypic variation that is associated
with these genotypes.
Ambystoma mexicanum is by far the best-studied member of the tiger salamander
species complex and has a long history as a model organism. It was originally brought
into laboratory culture by Aguste Duméril in 1870 (Duméril, 1870; Smith, 1989). Since
this time, the species has served as a model system for studying vertebrate development
(Armstrong and Malacinski, 1989), hence its other handle – the laboratory axolotl. Until
recently, however, extensive genetic analyses have been limited by several factors,
including: an exceedingly large genome size of roughly 30 gigabases (Gb) (Licht and
Lowcock, 1991), a relatively long generation time, and a relatively small number of
assayable genetic markers. That is not to say that genetic analyses are impossible for the
species. In fact, my predecessors have succeeded in identifying phenotypes that segregate
as Mendelian (single gene) factors (reviewed by Armstrong and Malacinski, 1989). Most
critical among these are the discoveries that: 1) sex segregates as a Mendelian locus,
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