Deciphering the swordtail’stale: a molecular andevolutionary questAdam S. Wilkins
SummaryThe power of sexual selection to influence the evolutionof morphological traits was first proposed more than130 years ago by Darwin. Though long a controversialidea, it has been documented in recent decades for a hostof animal species. Yet few of the established sexuallyselected features have been explored at the level of theirgenetic or molecular foundations. In a recent report,Zauner et al.(1) describe some of the molecular featuresassociated with one of the best characterized of sexuallyselected traits, the male-specific tail ‘‘sword’’ seen incertain species of the fish genus Xiphophorus. Zauneret al. find that the msxC gene, a gene previouslyimplicated in fin development from work in zebrafish, isdramatically and specifically upregulated in the deve-lopment of the ventral caudal fin rays, which give riseto the sword, in males. The results provide the firstmolecular insight into the development of this sexuallyselected trait while prompting new questions about thestructure of the entire genetic network that underliesthis trait. To fully understand the molecular-genetic andevolutionary history of this network, however, it will beessential to determine whether sword-development is abasal or derived trait in Xiphophorus. BioEssays26:116–119, 2004. � 2004 Wiley Periodicals, Inc.
Introduction
The concept of sexual selection, the idea that morphological
traits can evolve in response to mating preferences and/or
competition formates,was first put forward byCharlesDarwin,
in his book The Descent of Man and Selection in Relation to
Sex.(2) The hypothesis was intended to explain those striking
sexually dimorphic features in animals that cannot be readily
understood within the general adaptationist framework of the
theory of natural selection. The archetypal example of sexual
selection is the male peacock’s tail which confers no general
adaptive advantage, indeed a probable disadvantage for
survival, but which has utility in the male peacock’s wooing of
the female. In principle, all pronounced secondary sexually
dimorphic traits might be the products of sexual selection, and
serve either to enhance chances of mate selection or to
function in intra-sex competition for mates.
Although the idea of sexual selection, in particular Darwin’s
belief that female choice of mates on the basis of the
‘‘aesthetic’’ appeal of certain male traits can influence the
evolution of the latter, was treated for many decades with
skepticism, sometimes bordering on derision, the general
phenomenon of sexual selection and Darwin’s hypothesis
about the importance of female preference are now well
accepted.(3,4) Yet, the study of sexually selected traits has
remained, for the most part, at the purely morphological and
behavioral levels. In contrast to many features that are shared
equally between both sexes, such as external sensory organs,
appendages, various internal organs, segmentation patterns,
and bristle arrays, the analysis of sexually selected traits in
terms of their genetic or molecular foundations is still in its
comparative infancy. One of the few such studies of this kind
explored the basis of the control of sexually dimorphic pigment
patterns in an invertebrate group, the Drosophilids.(5)
In a recent report, Zauner et al.(1) have now extended the
study of sexual selection at the molecular level to the ver-
tebrates, specifically the fish genus Xiphophorus. The trait
studied is a classic one in the field of sexual selection—it was
noted by Darwin—and consists of the extension of the ventral
caudal fin-rays into a decorative ‘‘sword’’-like structure in the
male tail. Specifically, Zauner et al. analyzed sword formation
in juvenile males and in tail-fin regeneration with respect
to expression of a gene, msxC, that had previously been
implicated in fin-ray development in the zebrafish. They find
that msxC expression is elevated specifically in the ventral
caudal fin rays that give rise to the sword, both in normal
development and in fin regeneration.
This work will be briefly reviewed here and discussed with
respect to some of the wider genetic and evolutionary
questions and issues that it touches upon.
A short history of the Xiphophorus ‘‘sword’’
The Xiphophorus sword is a long extension of the ventral fin
rays that develops during sexual maturation of males, at
approximately 4–6 months. It is exclusively a male trait in
normal development, never spontaneously appearing in
116 BioEssays 26.2 BioEssays 26:116–119, � 2004 Wiley Periodicals, Inc.
BioEssays Editorial Office, 10/11 Tredgold Lane, Napier Street,
Cambridge CB1 1HN, UK. E-mail: [email protected]
DOI 10.1002/bies.10414
Published online in Wiley InterScience (www.interscience.wiley.com).
What the papers say
females. A typical male swordtail is shown in Fig. 1, along with
a swordless female. Not all sword-producing species, how-
ever, produce the impressive sword shown in the figure; the
males of some species produce relatively fore-shortened
swords.(6–8) Furthermore, not all Xiphophorus species pro-
duce swords; it is a trait found in some sub-clades and not in
others.(7,8) The sub-genera that do not produce swords are
designated as ‘‘platyfish’’ and sword production is evidently
a trait that has either been gained or lost more than once
within this genus (see below). Intriguingly, however, sword
production can be artificially induced both in females and in
some species that normally lack them, or in juvenile males
prematurely, by treatment with exogenous testosterone.(6)
Evidently, much of the genetic machinery for producing the
sword is present more widely within the genus than its normal
phenotypic manifestation would suggest.
Although the evolutionary history of the sword within
Xiphophorus is thus still somewhat murky, the evidence
that it is involved in sexual selection is strong. In particular,
female Xiphophorus have been shown to prefer males with
larger swords.(9) In this respect, swordtails are reminiscent of
peacocks; females of both groups seem to prefer exagge-
rated male traits, a possible instance of ‘‘runaway sexual
selection’’.(3) Furthermore, female platyfish that normally
never see swords in conspecific mates, are more inclined to
mate with sword-bearing males, when a sword has been
artificially attached.(10)
The evidence for a role of msxC in the
development of the male ‘‘sword’’
Since sword formation, or its absence, is a species char-
acteristic, the traitmust have a genetic basis. Several decades
ago, hybridization experiments between swordtails and platys
indicated that the trait was polygenic, involving several
unlinked genes.(6) The precise identity of the actual ‘‘sword
genes’’, however, remained unknown. For their molecular
analysis, Zauner et al. built on some previous observations on
fin regeneration in zebrafish. In that work, Akimenko et al.(11)
had reported that four genes of the msx (muscle segment
homeobox) subfamily of homeobox genes, zebrafish msxA,
msxB, msxC, andmsxD—a gene family previously implicated
in various epithelial-mesenchymal developmental processes
in vertebrates—are strongly upregulated during caudal fin
regeneration. All fourmsx genes show characteristic temporal
patterns and positional and tissue locations in the blastema of
the regenerating tail fin.
Zauner et al., therefore, decided to investigate msx
gene expression in relationship to sword development, in
both normally maturing Xiphophorus males of sword-bearing
species and in caudal fin regeneration, following amputa-
tion of the distal part of the fin including the sword, in those
species. They used RT-PCR to isolate msx homologs and
identified two, one an apparent msxC ortholog, the other
showing sequence relatedness to both Xenopus msx1 and a
fifth fish msx gene, msxE (which had been identified in both
zebrafish and puffer fish). The latter gene they designated
msxE/1.
The expression of msxC gives a clear and unambigous
correlation with sword development. Although it is expressed
at low but significant levels in the distal tips of all developing
caudal fin rays in normal development, it is expressed at much
higher levels specifically in the distal tips of the ventral fin rays
that will give rise to the sword. Furthermore, testosterone
treatment of juvenile males, a procedure that can prematurely
induce sword development, induced a small caudal protrusion
at 10 days which was accompanied by strong msxC expres-
sion in the developing sword rays specifically. In females,
in contrast, there was no elevation ofmsxC expression in any
of the developing rays during caudal fin regeneration. Similar
results were found in two sword-bearingXiphophorus species,
from different clades, X. helleri and X. montezumae although
the precise kinetics of sword development and of msxC
expression differed slightly between them. In both species,
two domains of expression were seen in regenerating fins:
Figure 1. Amale, sword-bearing Xiphophorus and a female.
(Photograph courtesy of Dr Manfred K. Meyer).
What the papers say
BioEssays 26.2 117
the distal, undifferentiated cells of the blastemata, and in a
more proximal mesenchymal domain that may correspond to
cells destined to become scleroblasts. The latter expression
domain during fin regeneration is not seen in zebrafish,(11) thus
marking one difference in fin development between the two
fish groups.
In a different pattern to msxC, expression of the msxE/1
gene showed detectable but lower levels of expression in the
future sword rays than in the middle or dorsal rays of re-
generating caudal fins. Furthermore, in data reported but not
shown by Zauner et al.,(1) msxE expression was not detected
at all in regenerating caudal fins of the zebrafish, a second
apparent difference between these two relatively distantly
related actinopterygians. The precise significance of this
observation, however, is difficult to evaluate.
Some developmental genetic
and evolutionary questions
These findings provide an important entry point into under-
standing the developmental basis of a sexually selected
morphological trait in a vertebrate species, probably constitut-
ing the first report of this kind. Yet, clearly, this is still only a
beginning. The results serve to highlight how much is still
unknown.
The first obvious question, of course, concerns the devel-
opmental role of the msx genes in fin development generally
and in sword fin ray development specifically. As homeobox
genes, msx genes are transcription factor genes and the
question transforms, in part, to one about the target genes and
processes that they govern. Given their predominant distal
expression in fin rays that have exaggerated distal growth,
one might surmise that they function as regulators of growth
outwards along the proximodistal axis. Intriguingly, the msx
genes are fairly closely related in sequence to another homeo-
box gene subfamily, the Distalless (Dlx) genes,(12) many of
whose members are involved in proximodistal patterning.
Expression data implicate FGFs and signalling through FGF
receptors as part of this process.(13,14)
Yet, such connection can only be part of the regulatory
story. Both themsx genes andFGF signallingmust be part of a
larger regulatory structure, a genetic network of some kind.
Since exposure to testosterone can trigger sword develop-
ment in females,(6) it must be that much of the network exists
in both sexes but is only activated by the testosterone signal
in male animals, which induces male secondary sexual
characteristics. Such considerations are connected to ques-
tions about the evolutionary origins of the network itself and
how its structure has been altered in different Xiphophorus
sub-clades. A fundamental question in this regard is whether
the sword has been gained in some lineages, relative to an
ancestor that lacked this appendage, or whether it was lost in
several lineages, in independent events, after divergence from
an ancestral stock that possessed swords.
Perhaps surprisingly, given the extent of phylogenetic
analysis given to this problem over the years, there is still no
firm consensus. The sister group of Xiphophorus, Priapella,
is swordless and the traditional interpretation is that sword
development in Xiphophorus was not ancestral but was
independently acquired several times within the group.(7) In
contrast to this view, Meyer et al.,(15) using molecular phylo-
genetic analysis in a report published in 1994, concluded
that sword-formation was an ancestral, autapomorphic trait of
Xiphophorus. In this interpretation, sword development has
been lost independently in several lineages within the genus.
Subsequent detailed analysis of both morphological and
molecular material has not, however, succeeded in resolving
the issue.(16) Nor do a priori arguments really help since either
pattern can be rationalized in selective terms. Multiple
instances of sword acquisition, for instance, would testify to
the value of this trait in sexual selection, as sword-bearing
males compete more favorably for females than swordless
ones. In contrast, multiple losses of the sword in evolution of
the genus could be explained in terms of a natural selective
‘‘cost’’ to sword possession.
To the extent that the different phylogenies are compatible
with either multiple gains or losses, however, both possibilities
would be instances of parallel evolution, the phenomenon in
which the ‘‘same’’ evolutionary change occurs independently
in several related lineages. As noted by Simpson(17) and by
Meyer,(18) parallel evolution almost certainly implies the
existence of unexpressed ‘‘genetic potential’’—genetic net-
works, in today’s terminology—that can be activated in related
lineages in a relatively simple fashion.
In this particular case, the apparent multiplicity of occur-
rences of the transition between sword development and
swordlessness within the genus implies that only a small
number of genetic changes are involved. In principle, multiple
loss—if sword-development is a basal trait(8,15)—could involve
mutations in several points of the network.On theother hand, if
the basal state was swordlessness, the ‘‘invention’’ of swords
independently in several lineages would, almost certainly,
involve comparatively minor changes, in the form of new links,
to a network that already existed and which was active in
another role. Indeed, Zauner et al. present some evidence
for the existence of a related, but slightly different, network in
the development of a second male morphological feature,
the gonopodium, a modified anal fin. They show that msxC
expression also takes place during (and is presumably
required for) the formation of the gonopodium. Indeed, from
the comparative probable respective evolutionary ages of
gonopodia and swords, the development of the sword may
have involved recruitment of some of the genetic machinery
of the former when the latter first evolved.(1) Ultimately, the
elucidation and comparisons of the various networks involving
msxC expression, both within fish and in other vertebrates,
will provide clues to the probable evolutionary relationships of
What the papers say
118 BioEssays 26.2
these networks and the relationships between these networks
and the specific phenotypic traits that they underlie.
Yet, leaving aside this longer-term and broader perspec-
tive, the work of Zauner et al. is, in itself, an important advance
in the study of sexual selection. It provides an important insight
into a classic instance of the phenomenon, the sword of the
Xiphophorus swordtail fishes. More generally, it illustrates
the potential to extend molecular-genetic analysis of devel-
opment to vertebrate sexually selected traits. Darwin would
surely have been pleased.
References1. Zauner H, Begemann G, Mari-Beffa M, Meyer A. Differential regulation
of msx genes in the development of the gonopodium, an intromittant
organ, and of the ‘‘sword’’, a sexually selected trait of swordtail fishes
(Xiphophorus). Evol Dev 2003;5:466–477.
2. Darwin C. 1871. The Descent of Man and Selection in Relation to Sex.
London: John Murray and Sons.
3. Andersson M. Sexual Selection. 1994. Princeton: Princeton University
Press.
4. Zuk M. Sexual selection. In: Pagel M, editor. Encyclopaedia of Evolution.
Oxford: Oxford University Press. Vol. II, pp. 1047–1051.
5. Kopp A, Duncan I, Carroll SB. Genetic control and evolution of sexually
dimorphic characters in Drosophila. Nature 2000;408:553–558.
6. Zander CD, Dzwillo M. Untersuchungen zur Entwicklung und Vererbung
des Caudalfortsatzes der Xiphophorus-Arten (Pisces). Zeitschr.
Wissensch. Zool 1969;178:275–315.
7. Rauchenberger M, Kallman KD, Morizot DC. Monophyly and geography
of the Rio Panuco Basic swordtails (genus Xiphophorus) with descrip-
tions of four new species. Am Mus Novit 1990;2975:1–41.
8. Meyer A. The evolution of sexually selected traits in male swordtail fishes
(Xiphophorus: Poeciliidae). Heredity 1997;79:329–337.
9. Basola AL. Female preference for male sword length in the green
swordtail, Xiphophorus helleri (Pisces: Poeciliidae). Anim Behav 1990a;
40:332–338.
10. Basola AL. Female preference predates the evolution of the sword in
swordtail fish. Science 1990b;250:808–810.
11. Akimenko MA, Johnson SL, Westerfield M, Ekker M. Differential
regulation of four msx homeobox genes during fin development and
regeneration in zebrafish. Development 1995;121:347–357.
12. Bendall AJ, Abate-Shen C. Roles for Msx and Dlx homeoproteins in
vertebrate development. Gene 2000;247:17–31.
13. Poss KD, Shen J, Nechiporuk J, McMahon G, Thisse B, Thisse C,
Keating MT. Roles for FGF signalling during zebrafish fin regeneration.
Dev Biol 2000;222:347–358.
14. Wang Y, Sassoon D. Ectoderm-mesenchyme and mesenchyme-
mesenchyme interactions regulate Msx-1 expression and cellular
differentiation in the murine limb bud. Dev Biol 1995;168:374–382.
15. Meyer A, Morrissey JM, Schartl M. Recurrent origin of a sexually selected
trait in Xiphophorus fishes inferred from a molecular phylogeny. Nature
1994;368:539–542.
16. Marcus JM, McCune A. Ontogeny and phylogeny in the northern
swordtail clade of Xiphophorus. System Biol 1999;48:491–522.
17. Simpson GG. Fossils and the History of Life. 1983. San Francisco: WH
Freeman.
18. Meyer A. 1999. Homology and homoplasy: the retention of genetic
programmes. In: Bock GR and Cardew G, editors. Homology. pp. 141–157.
Novartis Foundation Symposium 222. John Wiley & Sons: Chichester.
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