AMER. ZOOL., 36:24-35 (1996)
Phylogenetic and Mechanistic Analysis of A Developmentally Integrated
Character Complex: Alternate Life History Modes in
Ambystomatid Salamanders'
H. BRADLEY SHAFFER AND S. RANDAL VOSS
Center for Population Biology and Section of Evolution and Ecology, University of California,
Davis, California 95616
SYNOPSIS. Many of the critical pieces are now in place to exploit the
evolution of paedomorphosis (metamorphic failure) as a model for both
the phylogenetic and mechanistic analysis of a developmentally integrated
character. Phylogenetic analyses indicate that paedomorphosis has
evolved convergently among different populations and species of ambystomatids during the recent past. Mechanistically, there is growing evidence that paedomorphosis is controlled by a few loci, with different
combinations of loci and alleles blocking metamorphosis in different conspecific populations and among closely related species. A reductionistic
approach is needed to determine if the same or different loci were altered
in the independent evolution of paedomorphosis among different ambystomatid lineages. We provide an overview of the genetic strategies that
we are using toward this end, and present some of our own published
and unpublished results. We then briefly discuss the potential insights and
limitations of the reductionistic approach to character evolution that we
are advocating in this research strategy.
INTRODUCTION
When several, seemingly "different"
characters are integrated by common regulatory systems during development, we
consider them to comprise a developmentally integrated system (Hanken et al.
1989). Such characters, or character complexes, have been viewed as both a blessing
and a curse by evolutionary biologists. On
one hand, developmentally integrated characters are often viewed as evolutionarily
stable. This follows from the assumption
that developmental integration, once
achieved, is intrinsically stable to perturbation, because a change in a single component is not possible without compromising the regulatory system underlying the
entire character complex, or the expression
of all subsequent characters during ontogeny (Gould, 1982; Alberch 1982, 1985;
Maynard Smith et al. 1985). Under this
1
From the Symposium Historical Patterns of Developmental Integration presented at the Annual Meeting of the American Society of Zoologists, 4-8 January 1995, at St. Louis, Missouri.
24
view, developmentally integrated characters
may be ideal for both phylogenetic reconstruction and functional analysis. The alternative view is that the integrated nature of
such characters renders them inappropriate
as phylogenetic characters. Here, the objection stems from a perception that one cannot recognize which sets of characters are
free to evolve independently, and which are
not (Shaffer, 1986). The difference between
these views really amounts to identifying a
defensible strategy for identifying characters that are developmentally integrated,
and those that are evolutionarily independent. Having done so, one would presumably weight independent characters equally,
but would down-weight characters which
are constrained to evolve concordantly.
Are developmentally integrated character
complexes the friend or foe of the evolutionary biologist? Are they plagued with
problems of convergence, or are they so
stable that shifts in structure and function
are unique events that accurately trace evolutionary history? Is there a time frame to
the stability of such characters, with the ca-
ALTERNATE LIFE HISTORIES IN URODELES
pacity for evolutionary reversals declining
over time (Marshall et al., 1994)? How can
one determine, with or without experimental evidence, whether the number of supposedly independent characters has been
counted correctly? When one sees a large
number of unique synapomorphies at one
node of a tree, should one be satisfied that
a well-supported node has been discovered,
or suspicious that they are really parts of a
single, integrated character, and not phylogenetically independent (Shaffer et al.,
1991)? One strategy for answering these
questions is to use a model system in which
both the evolutionary history and mechanistic basis of character variation can be
traced during the course of phylogenetic
history (Kellogg and Shaffer, 1993). The ultimate goal of this strategy is to understand,
at least in a few cases, how genetics and
development interact to produce complex
character changes during cladogenesis. Although such a model system approach necessitates sacrificing taxonomic breadth in
favor of mechanistic depth (Hanken, 1993),
it provides the only real opportunity to evaluate, in detail, the consequences of developmental integration on character evolution. By carefully choosing appropriate
model systems that span different levels of
phylogenetic relationship, it may be possible to discern patterns of character evolution that transcend particular lineages, and
address the difficult issues of choosing and
weighting characters for phylogenetic analysis.
In this paper, we first discuss amphibian
metamorphosis as an exemplar for studying
a developmentally integrated character
complex. We then review the evolutionary
history of metamorphosis in the tiger salamander complex, arguing that this radiation
of approximately 15 species comprises an
appropriate model system in which to investigate the evolutionary history of this
highly integrated character. We present
some of our own published and unpublished results on the mechanistic basis of
metamorphic failure, (which we will refer
to as paedomorphosis in this paper), and the
evidence suggesting that there may be several genetic pathways by which this life history mode has evolved. Finally, we end
25
with a brief discussion of the reductionistic
approach to character evolution that we are
advocating in this research strategy.
METAMORPHOSIS: A MODEL OF
DEVELOPMENTAL INTEGRATION
Amphibian metamorphosis includes all
of the processes that mediate the conversion
of larval characters to adult characters. For
salamanders, metamorphosis involves numerous character changes, including the
loss of external gills and tail fin, restructuring of the skull and associated musculature
(Dodd and Dodd, 1976), and replacement
of larval with adult hemoglobin (Maclean
and Jurd, 1971; Ducibella, 1974), among
many others. In terms consistent with this
paper, all of these characters are developmentally integrated at metamorphosis, and
the entire suite of integrated characters defines a morphology that is qualitatively different from that of the larva. All of the
metamorphic processes that underlie developmental integration are mediated by a
common regulatory system. This regulation
involves a relatively well-characterized
physiological cascade of several thyroid
hormones and their target tissues (Fig. 1).
Therefore, metamorphosis is a highly coordinated set of processes, in which developmental integration of morphological
characters is mediated by a physiological
system of integrated regulatory hormones.
The typical mode of urodele development is metamorphosis; an aquatic larva
metamorphoses into a terrestrial adult.
However, some species have an alternate
developmental mode, which we simply refer to as paedomorphosis (see Gould [1977]
and Shaffer, [1984a] for some of the discussion associated with this terminology).
Paedomorphosis is characterized by metamorphic failure; paedomorphs remain in the
aquatic habitat and reproduce while retaining most of their larval morphology. Paedomorphosis may be obligate or variable at
different taxonomic levels. For example,
Proteidae, Cryptobranchidae, and Sirenidae
are obligate paedomorphic families (Duellman and Trueb, 1986); the genus Ambystoma (Ambystomatidae) is variable, although several Mexican species are obligate
paedomorphs (Shaffer, 1984a; Brandon,
26
H. B. SHAFFER AND S. R. VOSS
Thryoid Hormone Action
TRH
Hypothalamus
TSH
Pituitary
TH
Thyroid
nuclear receptors
Target cells ^monodeiodinase system
T4/T3 binding affinity
BRAIN
PERIPHERY
FIG. 1. A simplified diagram of the thyroid hormone cascade in mammals.
1989). Other species of Ambystoma and
Notophthalmus are facultative paedomorphs, in which either the metamorphic or
paedomorphic life history mode is possible;
this variation may exist within populations,
families, or ontogenetically within individuals (Sprules, 1974; Patterson, 1978; Semlitsch and Gibbons, 1985; Harris, 1987;
Licht, 1992).
At the most basic level, thyroid hormone
(TH) is essential for amphibian metamorphosis, and delivery of TH to peripheral
target cells at the proper time appears to be
one of the critical components of the system. In mammals, where the regulation of
TH has been studied most extensively (reviewed by Larsen, 1989; Williams, 1994),
TH synthesis and secretion is under the
control of the hypothalamus and pituitary
(Fig. 1). The control is hierarchical, with
the hypothalamus releasing neuropeptides
(Thyrotropin Releasing Hormone, or TRH)
that act at the pituitary. The pituitary then
synthesizes and secretes Thyroid Stimulating Hormone (TSH) that acts at the thyroid
to control TH synthesis and secretion. At
the peripheral target cells, 5'D deiodinase
(hereafter, 5'D) converts T4 into the more
active T3, whereas 5D deiodinase (hereafter
5D) deactivates TH.
In normal amphibian metamorphosis, the
cascade described above is well established
from the thyroid to the periphery (reviewed
by Galton, 1992a; Shi, 1994). After conversion of T4 into T3, this activated hormone acts through specific receptors to regulate expression of target genes, the prod-
ucts of which bring about metamorphosis.
However, "above" the thyroid level (that
is, to the left in Fig. 1), the system in amphibians is less well known. It is clear that
TSH is produced at the pituitary and is involved in the regulation of TH (reviewed
by Dodd and Dodd, 1976). However, at the
hypothalamus, the releasing hormones are
not well understood. It now appears that
TRH may not be the primary factor (Taurog
et al., 1974; Sawin et al, 1978, Darras and
Kuhn, 1983; but see Jacobs et al, 1988).
Instead, Corticotropin Releasing Hormone
(CRH) has been identified as a potential releasing factor (Denver, 1988, 1993; Denver
and Licht, 1989; Gancedo et al, 1992; but
see Jacobs and Kuhn, 1989). The interpretation of these experiments is still somewhat equivocal, and the results are based
upon diverse taxa and animals of different
ages. However, it seems likely that TRH,
CRH, or similar neuropeptides (Denver,
1993) may play a role in pituitary regulation of TSH.
MECHANISTIC BASIS OF PAEDOMORPHOSIS
Conceivably, any alteration of the metamorphic cascade, from hypothalamus to
thyroid hormone receptor, could lead to
paedomorphosis. Indeed, different physiological mechanisms have been altered in the
convergent evolution of paedomorphosis
among some major clades of urodeles. Several paedomorphic species (A. mexicanum
[Huxley, 1920]; Eurycea neotenes [Kezer,
1952]; Gyrinophilus palleucus [Dent et al.,
1955]; Typhlomoge rathbuni [Dundee,
ALTERNATE LIFE HISTORIES IN URODELES
1957]; A. dumerilii [Brandon, 1976]) were
observed to undergo metamorphosis after
treatment with thyroid hormones. These
salamanders apparently have a functional
metamorphic pathway in the presence of
sufficient T4. Thus, paedomorphosis is not
caused by a failure to respond to T4, but
rather to some mechanism further "up" the
hormonal cascade (to the left of TH in Fig.
1). In contrast, Necturus and Proteus (family Proteidae) do not undergo metamorphosis, even after large doses of T4 (Lynn,
1961; Dent, 1968). In these species, obligate paedomorphosis has apparently
evolved by changes further "down" the
cascade that are involved with the modification and regulation of T4. This lack of
response to T4 could be due to the amounts
of 5'D or 5D, the numbers of thyroid hormone receptors in the nuclei of target cells,
or receptor affinities for T3 and T4 (Galton,
1985, 1992b). A further level of complexity
arises with those salamander species that
exhibit a facultative life history response,
where some individuals metamorphose and
others do not. Here, the expression of paedomorphosis or metamorphosis apparently
involves some type of interaction between
environmental factors and genetic control
mechanisms (Semlitsch and Gibbons, 1985;
Harris, 1987; Harris et al., 1990; Semlitsch
et al., 1990; Licht, 1992; Voss, 1996).
A spectacular example of variation in life
history pathways is seen among Mexican
and U.S. species of the Ambystoma tigrinum complex, where both obligate and facultative paedomorphosis have evolved several times during the last few million years
(Shaffer, \984a,b, 1993). The typical, and
apparently ancestral, life cycle for this
group is characterized by complete metamorphosis. The paedomorphic taxa differentiate rapidly both at the molecular (Shaffer, 1984a) and morphological (Shaffer,
1984&) levels, and their genetic isolation
from other, transforming populations apparently leads to a high likelihood for speciation compared to their metamorphosing relatives (Shaffer and Breden, 1989). This
group of salamanders thus offers a unique
opportunity to examine the mechanistic basis of variation in a tightly integrated character, during several independent iterations
27
from an ancestral transforming or facultative paedomorphic condition to a derived
obligatorily paedomorphic condition. In
particular, it allows us to pose the general
question: Have similar or different physiological and genetic mechanisms been altered in the independent evolution of paedomorphosis among Mexican ambystomatids? If only a single mechanism is
repeatedly involved, this implies that integrated characters with complex control may
be constrained to a single viable mechanism
of change. If so, it implies that convergence
can be complete even at the most reductionistic levels, and that such convergence
can occur in complex, developmentally integrated characters.
It is important to keep in mind that,
throughout this paper, we consider the
metamorphosis-to-paedomorphosis transition to represent the evolution of an integrated character complex. An alternative
way to view this system is that paedomorphosis represents the breakdown of an integrated system, because the component elements that change in a coordinated fashion
at metamorphosis are now free to vary independently. Because metamorphosis is apparently the ancestral character state in Ambystoma, we cannot reconstruct the sequence whereby metamorphic characters
became integrated by the same regulatory
system. We do not, and cannot know if
multiple characters were simultaneously integrated or if each character was integrated
independently. However, a novel approach
to this type of problem would be to examine the degree of integration of various
components of metamorphosis in different
obligate and facultative paedomorphs. For
example, although morphological metamorphosis does not occur in paedomorphic species, some biochemical traits are responsive
to the low levels of circulating thyroid hormone and undergo transformation. A transition in hemoglobin occurs at the same
time that morphological metamorphosis
would be expected to occur in the paedomorph A. mexicanum, exactly as it does in
transforming A. tigrinum (Ducibella, 1974).
In metamorphosing species, hemoglobin
transition might be functionally significant
in terms of terrestrial adaptation {e.g., Nuss-
28
H. B. SHAFFER AND S. R. VOSS
baum, 1976), as is indicated by the ontogenetic synchrony of this biochemical transition with other morphological components of metamorphosis. However, at the
level of hormonal control mechanisms in
the aquatic axolotl, hemoglobin (and other
blood tissue components) and morphological components of metamorphosis are not
tightly integrated. (Hanken et al., 1989 use
an experimental induction approach to
make the same argument for cranial elements in the anuran Bombina orientalis.)
Rather, hemoglobin metamorphosis follows
the ancestral ontogenetic schedule, but morphological metamorphosis does not. This
implies that different aspects of metamorphosis show different degrees of developmental integration, and the potential for different evolutionary histories. Analyses of
the degree of character dissociations in several paedomorphic species may provide insight into the evolutionary history of developmental integration, as well as the potential for future evolution after characters
become disintegrated from the same regulatory system.
ALTERNATE LIFE HISTORY MODES IN THE
TIGER SALAMANDER COMPLEX:
CONVERGENCE OR PARALLELISM IN A
DEVELOPMENTALLY INTEGRATED SYSTEM?
The tiger salamander complex, as defined
here, consists of about 15 species. Of these,
Ambystoma tigrinum is widely distributed
across most of the continental United States
and northern Mexico, and the remaining
species are narrowly-to-broadly distributed
throughout northern and central Mexico
(Shaffer, 1984a; Brandon, 1989; Shaffer
and McKnight, 1996). During the past 10
years, the phylogenetic relationships of
members of the tiger salamander complex
have received considerable attention, primarily using allozyme (Shaffer, 1984a;
Jones et al., 1988) and mtDNA (Shaffer and
McKnight, 1996; Routman, 1993) data sets.
One of our primary foci in this work has
been to construct phylogenetic hypotheses
that will allow us to reconstruct the pattern
of life history transitions between metamorphosis and paedomorphosis during the
phylogeny of the tiger salamander complex.
Two analyses of phylogenetic relation-
ships have sufficient taxonomic breadth to
map different life history modes: an allozyme analysis of the Mexican members of
the complex (Shaffer, 1984a, 1993), and a
more complete mtDNA analysis of 77 populations, including virtually all of the recognized taxa and all known cases of obligate paedomorphosis (Shaffer and McKnight, 1996). These two data sets provide
complimentary, and not necessarily congruent, insights into the phylogeny of the
group. However, they both imply that paedomorphosis has been an extremely labile
character that has evolved multiple times
within this lineage.
The allozyme data include most of the
Mexican members of the tiger salamander
complex: using the western U.S. species A.
gracile (a distant outgroup) to root the tree,
it provides a reasonably complete taxonomic sampling of 32 loci for 22 populations or
species of ingroup taxa. Based on several
different methods of phylogeny reconstruction, paedomorphosis has evolved several
times during the history of the group (Shaffer, 1984a, 1993). Exactly how many times
depends on the interpretation of certain internal nodes of the phylogeny (and some
are relatively weak, based on maximum
likelihood confidence intervals [Shaffer,
1993]) and on the resolution of equivocal
internal branch reconstructions (Shaffer,
1993). When populations are categorized according to three life history modes (obligatorily transforming, paedomorphic, or facultative), parsimony mapping (MacClade:
Maddison and Maddison, 1992) requires a
minimum of nine evolutionary transitions
among life-history modes during the evolution of 23 taxa (Shaffer, 1993). The precise
history of transitions among life history
states cannot be reconstructed with certainty;
for example, of the 86 equally parsimonious
resolutions of the reconstruction shown in
Shaffer (1993), 29 reconstruct obligate paedomorphosis as evolving twice, 45 require
three originations, and 12 require 4 independent originations.
Mapping life history condition onto the
mtDNA data set is also problematic, in
large part because of the low bootstrap P
values for many internal nodes, and thus the
relatively low confidence in any individual
29
ALTERNATE LIFE HISTORIES IN URODELES
Metamorphosis
unordered
transforming
paedomorphic
facultative
equivocal
FIG. 2. A reconstruction of the evolution of life history mode in the tiger salamander complex. Life history
was mapped using MacClade 3.0 (Maddison and Maddison, 1992) onto a majority rule consensus tree of 1450
equally parsimonious trees, based on 840 base pairs of the mitochondrial D-loop and an adjacent intron (modified
from Shaffer and McKnight, 1996). Metamorphosis is treated as an unordered character with three states: transforming, facultative (both conditions found in a single population), and paedomorphic. Taxon names are the
species or subspecies of Ambystoma, followed by the general locality of the sample; for exact localities, see
Shaffer and McKnight (1996). The taxon "t. nebulousm" SSC refers to the "South Central Colorado" haplotype
in Shaffer and McKnight (1996).
tree reconstruction. In Figure 2, we have
used a trimmed version of the majority rule
consensus tree from Shaffer and McKnight
(1996), in which we retained all of the
Mexican populations plus single representatives of the major U.S. tiger salamander
clades. This sampling strategy provides a
lower estimate of the number of state transitions between metamorphic and facultative paedomorphosis than would occur if all
of the U.S. populations were included, since
many of the great plains and rocky mountain populations in the subspecies diaboli,
melanostictum, mavortium, and nebulosum
are facultative paedomorphs (Collins et al.,
1980). However, none of these populations
are fixed for obligate paedomorphosis, and
so this life history variation will not effect
evolutionary reconstructions of the obligate
condition. Depending on whether polyto-
mies are considered to be "hard" (that is,
they represent simultaneous speciation
events, with each descendent independently
inheriting its life history condition from an
immediate common ancestor) or "soft"
(where a polytomy is interpreted as an unresolved series of bifurcation events, [Maddison and Maddison, 1992]), this tree requires a minimum of 14 or 13 changes
among life history modes, respectively, and
a minimum of four independent evolutions
of obligate paedomorphosis (Fig. 2).
Several important conclusions emerge
from this phylogenetic mapping exercise.
First, within the tiger salamander complex,
paedomorphosis has evolved several times.
Thus, under a phylogenetic definition,
many instances of obligate paedomorphosis
are not homologous, at least for several of
the more distantly-related lineages. Second,
30
H. B. SHAFFER AND S. R. VOSS
although it represents a complex and inte- Tompkins (1978) reported that a single regrated character complex, life history mode cessive allele in a homozygous condition
is evolutionarily labile. If this result gen- determined paedomorphosis with respect to
eralizes to other integrated systems (e.g., transforming A. tigrinum tigrinum. HowJeffery and Swalla, 1991, 1992; Raff, 1987; ever, in a recent test of this hypothesis, Voss
Raff et al., 1991), it suggests that devel- (1996) found that at least a two gene model
opmental complexity and integration per se was required to explain paedomorphosis in
provide little in terms of a priori expecta- this species (Fig. 3).
tions on character stability, and thus charTwo different strategies have been used
acter weighting. Even for these complex to investigate the genetic mechanisms that
characters, stability and phylogenetic utility underlie life history mode. Both utilize traare empirical issues that must be deter- ditional (non-molecular) approaches to inmined on a case-by-case basis. Third, even fer aspects of the genetics of metamorphoif it is considered as a unitary character sis. Semlitsch and Gibbons (1985) found
(rather than a large number of "indepen- that propensity for life cycle mode is popdently" evolving characters [Shaffer, ulation-specific in the facultative paedo1986]), life history mode may be a partic- morph A. talpoideum, and therefore apparularly uninformative character that is ently has a genetic basis. Furthermore,
plagued with large levels of homoplasy.
when within and between population crosses
of this species were compared, different
At this point, many investigations might
dismiss characters like metamorphosis and patterns of metamorphosis and paedomorpaedomorphosis; while they may be devel- phosis were found for some combinations
opmentally interesting, they are too homo- (Harris, et al., 1990). This suggested that
plasious to be of phylogenetic importance. different combinations of alleles and/or loci
However, salamander life history modes of- may underlie variation in life history mode
fer a unique opportunity to examine, in between nearby populations.
mechanistic detail, the way in which conAt a different taxonomic level of invesvergence has evolved. In particular, does tigation, Shaffer (1993; unpublished data)
the convergence we find at the phylogenetic used a genetic complementation approach
level represent different mechanisms at the to examine the genetic basis of life history
physiological or molecular levels leading to mode among three Mexican ambystomatids
the same phenotype, or have the same (A. mexicanum, A. dumerilii, and the Lamechanisms evolved in parallel in each lin- guna La Preciosa population of A. velascf).
eage. Although we are still far from a com- When individuals of these obligate paedoplete answer to this question, we review be- morphic taxa were reciprocally crossed, a
low the currently available data, and direc- proportion of the larvae from each of the
tions for future investigations with this sys- inter-specific crosses metamorphosed, pretem.
sumably reflecting the segregation of different alleles or different loci responsible
GENETIC BASIS OF PAEDOMORPHIC LIFE
for paedomorphosis in these three species.
HISTORY MODES
This suggests that different mechanisms unThe genetic basis of life history mode derlie paedomorphosis in three closely-rehas been studied for several ambystomatids lated species, and that metamorphosis is unand one salamandrid. Some of the most in- der some form of polygenic control. Voss
tensively studied systems have been those (unpublished data) used a similar genetic
with facultative paedomorphosis. Studies of complementation approach to examine the
the newt (Notophthalmus viridescens) and mechanistic basis of paedomorphosis
two ambystomatids (A. gracile, A. talpo- among A. mexicanum and A. tigrinum maideum) indicate that facultative paedomor- vortium. This cross yielded all paedophosis has a polygenic basis (Harris, 1987; morphs, suggesting the interesting possibilSemlitsch et al., 1990; Licht, 1992). In con- ity, that the same genetic mechanism was
trast, paedomorphosis in A. mexicanum was altered in the evolution of paedomorphosis
initially thought to have a single gene basis: in these taxa.
ALTERNATE LIFE HISTORIES IN URODELES
31
While these studies provide an important
initial step in unraveling the population and
A. mexicanum _ ^ A. t. tigrinum
quantitative genetic basis of paedomorphosis, they do not allow one to identify specific
loci, and thus to interpret the mechanisms
of convergent evolution. To identify specific loci we advocate both physiological and
genetic approaches at the molecular and endocrine levels. Central to our strategy is a
crossing design in which interspecific crosses are made between paedomorphic and
metamorphic species (Voss and Shaffer,
(P/pi
1996). Although a salamander genetics program is a labor intensive enterprise (sexual
maturity is 1.2-1.8 yrs), this strategy allows
for a test of single gene inheritance (Voss,
1.00
1996), increases the level of polymorphism
that segregates among hybrid offspring
(Voss, 1993), and allows for the comparison
of physiological and genetic data of different paedomorphic species within the same
metamorphic background. In addition, this
crossing design can be used in combination
with powerful molecular approaches to attribute phenotypic variation at the organismal level to the inheritance of specific
genes or anonymous DNA markers (Michelmore et al., 1991; Lisitsyn et al., 1993;
Rosenberg et at, 1994; Tanksley et al.,
0.00
1995). Voss (1994) used a PCR based stratLow Food
High Food
Low Food
High Food
Low Temp
Low Temp
High Temp
High Temp
egy called bulked segregant analysis to
Treatments
identify a randomly amplified polymorphic
FIG. 3. The genetic basis of life history pathway DNA (RAPD) that was linked to one of the
was examined for the Mexican axolotl (A. mexican- genes underlying paedomorphosis in A.
um). Paedomorphosis in this species was thought to mexicanum. More recently, a second RAPD
have evolved by modification of a single allele (P —>
p) at a locus controlling metamorphosis (Tompkins, marker has been identified for this locus,
1978). This hypothesis was tested by crossing A. mex- and species-specific polymorphisms have
icanum to an obligate metamorph, A. tigrinum tigri- been identified for candidate genes (for exnum, and examing the segregation of life history phe- ample, thyroid hormone receptor) of known
notypes among the Fl generation, and among off- physiological function (Voss, unpublished
spring of two F2 backcrosses (Crosses 3 and 4). Offspring from these crosses (N = 275) were reared under data). Thus, it now seems feasible to dedifferent treatment combinations of food and temper- velop molecular markers for the specific
ature. All 122 of the Fl offspring metamorphosed, a genes that control life history mode, and to
result consistent with the single gene hypothesis. For use these markers to determine whether the
the F2 generation, the proportions of metamorphs and same or different genes are responsible for
paedomorphs among 3 treatment combinations (Treatment 1 = Low Food, Low Temperature; 2 = High convergence at the phylogenetic level.
Food, Low Temperature; 3 = Low Food, High Temperature) were also consistent with a single gene hypothesis, however proportions of metamorphs and paedomorphs in treatment 4 (High Temperature, High
Food) were significantly different from the expected 1:
1 Mendelian ratio. These data suggest that more than
one gene underlies metamorphosis in A. mexicanum,
and at least one gene is affected by the environment
(Voss, 1996).
WILL INCREASINGLY DETAILED ANALYSIS
PROVIDE THE "ULTIMATE" SOLUTION TO
THE HOMOLOGY PROBLEM?
Suppose we could find, clone, and sequence the entire set of genes controlling a
developmentally integrated character com-
32
H. B. SHAFFER AND S. R. VOSS
plex such as metamorphosis (see Wang and
Brown, 1991, 1993; Shi, 1994 for an interesting approach to this problem). Would
this ultimate level of reductionism allow us
to unambiguously distinguish homology
from homoplasy? If not, what would such
an approach accomplish?
The answer to this problem lies, in part,
in one's concept of homology (and thus of
convergence and homoplasy). If a character
is inferred to be homologous based on a
parsimony reconstruction of the taxa involved, and if one adheres absolutely to a
phylogenetic interpretation of homology,
then an understanding of underlying mechanisms cannot affect this interpretation. According to this way of thinking, homology
rests solely on whether or not character
states are derived from the same common
ancestor or different ancestors. A mechanistic understanding of a character complex
will only tell us at what level homoplasy
actually exists. As we pursue this approach,
we envision at least two potential outcomes.
It may be that Hennig's (1966) intuition
was correct; convergence often represents
misscored characters, and an increasingly
reductionistic view will uncover the true
differences among non-homologous states.
If this is the case, then the power of understanding the genetic basis of character evolution is obvious, for it provides a test of
theories of homology. Alternatively, it may
be that convergence is "real," at least in
the sense that the same mutations evolve
multiple times in different lineages. In this
case, one of the important messages from
the mechanistic research would be that homoplasy cannot be "cleaned up" by a more
thorough examination of characters. Rather,
it is an aspect of evolution that must be incorporated into our methods of phylogeny
reconstruction.
An alternative interpretation of complete
identity of two characters is that such a situation provides strong evidence of common
descent, and that one's original phylogeny
is incorrect. That is, "We may judge independent origin of the characters in the two
populations unlikely on the basis of detailed
similarity . . ." (Roth, 1994, p. 327), and
this may be particularly true for structurally
complicated characters ". . . because it
would seem more plausible for them to
have been maintained in all their complexity through continuity and replication, than
that they could arise independently more
than once by chance. Multiple origin is easier to imagine for a trait with only a few
fairly simple features . . . " (Roth, 1994, p.
328).
The resolution of such situations will
presumably depend on the strength of both
one's phylogeny reconstruction, and on an
assessment of the likelihood that such convergence at the DNA sequence level is
plausible. If the phylogenetic relationships
are solid, then the inference of homoplasy
presumably must be accepted. If the phylogeny itself is weak, then the preferred interpretation may be that all lineages with a
common mechanism underlying paedomorphosis are a monophyletic group, and that
the phylogeny has not been reconstructed
correctly. This may also hinge on whether
the information on metamorphic control is
itself used in the reconstruction of the phylogeny (a practice which many researchers
view as a circular, if a character that is used
in cladogram construction is then interpreted in light of that cladogram), or it may
amount to a form of a priori character
weighting (if one feels that such convergence at the molecular level is extremely
unlikely). However, in the absence of any
definitive studies, we can only say that the
answer to these questions is attainable, and
should provide fascinating new insights into
our understanding of how homoplasy
evolves in integrated characters.
ACKNOWLEDGMENTS
We thank V. Galton, M. McKnight, and
D. Heckel for their insightful insights, and
a cast of thousands who helped with salamander care on both coasts. H.B.S. was
supported by NSF (BSR 90-18686, DEB
93-06633) and the UC Davis Agricultural
Experiment Station. S.R.V. was supported
by NSF Dissertation Improvement Grant
BSR-9101128, a NSF-EPSCOR Grant to
the state of South Carolina, and an R. C.
Edwards Fellowship while at Clemson University. Travel costs were defrayed in part
by DEB 9406574 to M. Zelditch.
ALTERNATE LIFE HISTORIES IN URODELES
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