Xiphophorus helleri Heckel, 1848

KALLMAN, KLAUS D., WALTER, RONALD B., MORIZOT, DONALD C. & KAZIANIS, STEVEN, 2004, Two New Species of Xiphophorus (Poeciliidae) from the Isthmus of Tehuantepec, Oaxaca, Mexico, with a Discussion of the Distribution of the X. clemenciae Clade, American Museum Novitates 3441, pp. 1-34 : 16-19

Xiphophorus helleri Heckel

(1848; Green Swordtail) Figure 1G, H

DIAGNOSIS: A medium to large swordtail with a long straight caudal appendage. Midlateral stripe may be dusky or brownish (northern populations) or red. Two additional reddish stripes may be present above midlateral line and one beneath. Terminal segment of gonopodial ray 3 produced into a crescentshaped hook and blade pointed distally (fig. 3). Ray 4a curves strongly backward over the blade at an angle greater than 90°. Distal serrae of ray 4p reduced in size and number and proximal serrae rather slender. Terminal segment of ray 5a produced into a claw, several times larger than the distal serrae of ray 4p (fig. 3).

DISTRIBUTION: In coastal plain and front ranges. From Rio Nautla, Veracruz, Mexico, to northern Honduras (Rosen, [1960: fig. 7A, appendix 1] provided data for the upper Rio Coatzacolacos drainage).

MATERIAL EXAMINED: From Oaxaca: Rio del Sol, IBUNAM 13102 (2); Rio Junapan, IBUNAM 13103 (3), IBUNAM 31104 (8) UANL 15251 (3), AMNH 211356 (14); from Veracruz: Rio Coachapa, UANL 15252.

COMMENTS

Within both sexes of the four species, significant pairwise differences were discovered in indexed measures of body and caudal peduncle depth, dorsal fin base, predorsal length, eye to snout and eye to ventral distance, head length, orbital diameter, and anal fin length (tables 1, 2). Several of these characters exclusively distinguished the three taxa of the clemenciae clade. Body depth and caudal peduncle depth indices are significantly different for all three forms. X. monticolus possesses a rather narrow body and caudal peduncle; X. clemenciae exhibits the deepest body shapes while X. mixei is intermediate. Dorsal fin base length is significantly shorter in X. mixei and X. monticolus than in X. clemenciae . Predorsal length is longest in X. mixei and differs significantly from the two other species in males (only from X. clemenciae in females). The two newly described species differ significantly from X. helleri in body depth, length of dorsal fin base, and eye to ventral distance. Additional significant differences exist in caudal peduncle depth and mandibular length indices in males and head length and orbital diameter indices in females.

The sword of X. mixei is significantly shorter than those of the other taxa. The sword develops at the time of sexual maturation and continues to grow for several more months thereafter. The age of the mature males caught in the field is not known. It could be argued that all the mature X. mixei males had just attained maturity and that, therefore, the swords had not yet attained their final size. This is unlikely because many additional X. mixei males that were caught in the field and released all had short swords. Moreover, a stock of X. mixei has been bred in the Xiphophoru s Genetic Stock Center for several generations and males were maintained for at least one year past maturity. All males developed short swords, adding confidence to our conclusion from data derived from field collections. The sword of X. clemenciae is also significantly broader at its base than that of the other species, recalling its greater body and caudal peduncle depth.

The two newly described taxa and X. clemenciae possess gonopodia that are similar in structure. These similarities are present in studied structures within rays 4a, 4p, and 5a (table 3). There are, however, consistent differences that concern the spines on ray 3. The more distal spines in X. clemenciae are facing anteriorly, whereas in all 14 specimens of X. mixei and X. monticolus the distal spines are angled toward the more proximal. Because the most proximal spines of ray 3 are thick and angled toward the medial spines, in the two newly described taxa the spines are produced into a somewhat trian­ gular structure whereas in X. clemenciae they form a broader, more comblike feature. Ray 4a (also called the ramus) usually ends bluntly (11 out of 14 gonopodia), but in three gonopodia (one from each taxon) the small terminal segment angled sharply anteriorly, a condition previously illustrated for X. clemenciae ( Rosen, 1960) . The last segment of ray 5a of all three species is differentiated into a small claw that is approximately of the same size as the distal serrae on ray 4p.

PHYLOGENETIC RELATIONSHIPS

Several traits unite X. clemenciae , X. monticolus , and X. mixei and set them apart from the other swordtails. The orange lateral stripes of the three species are produced by carotenoid pigment, whereas the pigment responsible for the red stripes of the helleri clade is drosopterin ( Kallman and Bao, 1987).

A second diagnostic trait for the three species concerns the spots which develop in the proximal portion of the caudal fin. Every adult male exhibits red ( X. clemenciae ) or black ( X. mixei , X. monticolus ) spots, a trait not seen in any other Xiphophorus . The black spots in the proximal portion of the caudal fin of X. mixei and X. monticolus develop from the reticulum, bands of melanophores that border each scale pocket. When the males are exposed to agents that cause melanosomal (pigmentary) aggregation, orange spots are revealed. Thus with respect to this character the difference between X. mixei and X. monticolus on the one hand and X. clemenciae on the other is that in the two former species the melanosomes are invariably in the dispersed state and obscure the orange pigment cells, whereas in X. clemenciae the situation is reversed.

A third pigmentary trait is more difficult to evaluate. It concerns the axillary stripe, a band of melanophores extending from below the base of the pectoral fin to beyond the base of the gonopodium and into the caudal peduncle, one scale row above the midventral line. It is present in all adult males of the clade and expressed as a continuous zigzag line (at least in front of the gonopodium) following closely the reticulum. The only other Xiphophorus in which a similar pattern is present are adult males of the northern X. helleri populations (Rio Nautla, Rio Chachalaca, Rio Antigua, Rio Jampa systems; Rosen, 1960; Meek, 1904). In the Xiphophorus Genetic Stock Center , a strain derived from the Rio Chachalaca shows an axillary stripe that is produced as a row of spots rather than a continuous band, and it does not extend into the caudal peduncle. The same condition is exhibited in a male from the Rio Jamapa as illustrated in figure 21A by Rosen (1960), and figure 22 by Zander (1967). Because none of our other data reveals a close phylogenetic relationship between the clemenciae clade and X. helleri , we consider the trait(s) as being independently derived or, alternatively, the resemblance may be superficial. According to our observations on the Rio Chachalaca stock, in which two kinds of males occur, some with and some without this pattern ( Zander, 1967), presence or absence of this trait appears to be controlled by a recessive sex­linked allele. Because no variation is present within the clemenciae clade, no genetic analysis can be easily performed. F 1 hybrids between X. clemenciae and X. helleri (Sarabia stock) do not exhibit an axillary stripe. In many backcross hybrids to clemenciae the axillary stripe is poorly expressed (Kallman, unpubl. ms.), suggesting that the genetic control of the axillary stripe in the two species may not be the same. Such a situation would be analogous to the presence of the Sc (spotted caudal) pattern in X. variatus , a platyfish, and X. cortezi , a northern swordtail ( Rauchenberger et al., 1990).

MOLECULAR PHYLOGENY

A total of 71,933 total bases of genomic DNA sequences were derived from two nuclear genes (RAB27 and CCND1) in 29 fishes. This approach yielded 2359 nucleotide positions/characters that were conserved between all taxa. In addition, 153 were variable characters and, of these, 57 were informative for parsimony analyses. Data analysis revealed that X. clemenciae as well as the newly described species X. mixei and X. monticolus are unambiguously monophyletic. This conclusion is based on examination of phenograms/dendrograms derived from phenetic (neighbor­joining), parsimony, and maxi­

TABLE 4 Jukes and Cantor Corrected Distance Matrix Between Exemplative Individuals mum likelihood methods of phylogenetic reconstruction. A phylogenetic tree is provided (fig. 5) which shows bootstrap values derived from each of the three algorithmic methods. Using the parsimony method, strict consensus of 1000 reconstructed trees revealed absolute support for monophyly between X. clemenciae and the newly described taxa (data not shown). X. clemenciae individuals utilized in the analysis were specifically chosen to provide coverage of its entire geographic range. It is noteworthy that X. clemenciae individuals show more divergence from each other than do those of X. mixei and X. monticolus . This phenomenon is probably associated with the geographic ranges of these two newly identified species, which are relatively restricted compared to that of X. clemenciae , although this assumption is not proven by the data.

The clemenciae clade is also clearly more related to those included platyfish ( X. maculatus , X. couchianus , and X. andersi ) species and the included northern swordtail ( X. montezumae ) taxon than to X. helleri , X. alvarezi , and X. signum , as evidenced by bootstrap values (100%, 99%, and 97%, depending on the method of reconstruction; see fig. 5). Evidence for X. mixei and X. monticolus being distinct from other Xiphophorus taxa is supported by the phylogenetic reconstruction depicted in figure 5, which shows strong support for monophyly between all included individuals, regardless of geographic position. The genetic distance between species in the clemenciae clade, which includes the newly described taxa, are similar to distances between X. montezumae and X. maculatus for example, taxa of ‘‘northern’’ swordtail and platyfish clades, respectively (table 4).

The position of X. clemenciae itself within phylogenetic trees involving members of the genus has been the topic of debate in the literature ( Morizot and Siciliano, 1982; Rosen, 1979; Meyer et al., 1994; Borowsky et al., 1995; Meyer and Schartl, 2002). Although this study does not completely address the exact taxonomic relationship of X. clemenciae and the two newly described species within the genus Xiphophorus , it clearly shows with statistical confidence that these species are not closely related to the helleri clade. Undoubtedly, further inclusion of oth­ er Xiphophorus taxa and further data generation will address this research topic.

Support for monophyly of X. helleri , X. alvarezi , and X. signum is strongly supported by the data. The recently described X. mayae was not available and not included in the analysis, but presumably would also show close phylogenetic relationship to the ‘‘ helleri clade’’ taxa ( Meyer and Schartl, 2002). The common green swordtail, X. helleri , has the widest distribution of any Xiphophorus . The three other members of the helleri clade are found in Guatemala, suggesting that some populations in the eastern part of its range diverged into new forms. The similarity of the DNA sequences from both gene loci of X. alvarezi and the Lancetilla stock of X. helleri argues for a recent divergence from a common X. alvarezi / X. helleri ­like ancestor. Moreover, the apparent closer phylogenetic relationship of X. alvarezi , and perhaps also X. signum , to some (eastern) X. helleri populations than to others (Jalapa) has interesting taxonomic implications. If a similar relationship should be revealed by analyses of additional characters, then what is now known as X. helleri is really a paraphyletic assemblage of populations. This recalls an earlier description of the Jalapa population of swordtails as a separate species ( Meek, 1904). In addition, extremely high levels of allozyme diversity within and among the helleri clade was previously reported ( Morizot and Siciliano, 1982), providing additional evidence for genetic differentiation within this group of fishes.

Undoubtedly, further examination of the clemenciae and helleri clades and all Xiphophorus taxa will be required to fully address their exact evolutionary positioning. This study, however, does provide robust support that X. clemenciae and the two closely relat­ ed species, X. monticolus and X. mixei , do not belong to the evolutionary grouping that includes X. helleri , which supports the conclusions of other studies ( Morizot and Siciliano, 1982; Rosen, 1979; Meyer et al., 1994; Borowsky et al., 1995; Meyer and Schartl, 2002).

Figures

Fig. 1. Xiphophorus fishes of the Coatzacoalcos River System. A, B. X. mixei female and male. C, D. X. monticolus female and male. E, F. X. clemenciae female and male. G, H. X. helleri female and male. I, J. X. maculatus female and male.