Contributions to Zoology, 74 (3/4) (2005)J. Alexandrino; N. Ferrand; J.W. Arntzen: Morphological variation in two genetically distinct groups of the golden-striped salamander, Chioglossa lusitanica (Amphibia: Urodela)

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Discussion

Morphometric variation in the endemic Iberian salamander Chioglossa lusitanica was found to be consistent with documented genetic differentiation. Group 1, from south of the Mondego river in Portugal, is characterised by shorter digits than group 2, from north of the Mondego. A pattern of south to north clinal variation with increasing limb, toe and finger length was found superimposed on this dichotomy, resulting in stepped clines for each of the characters describing appendage size. We suggest that both historical, vicariant isolation and selection processes account for the observed variation. Short appendages, with a low volume to surface ratio, may represent an adaptation to xeric (i.e. less moist) environments (Nevo, 1972; Lee, 1993). Chioglossa lusitanica is a terrestrial streamside salamander extremely dependent on moist habitats and indeed the level of annual precipitation is the main predictor of its range in Portugal (Teixeira et al., 2001; Arntzen and Teixeira, 2006). Given that southern populations appear to occupy a more xeric environment than northern populations (Arntzen and Alexandrino, 2004) and assuming that rainfall gradients in the past paralleled those of today, selection could have produced the documented (stepped) clines. The general pattern of contemporary restricted gene flow between populations of C. lusitanica recently revealed by microsatellite data (F. Sequeira et al., unpublished data) would favour local adaptation along selection gradients, originating the observed morphological pattern of latitudinal variation. The action of either selection or stochastic drift, or both, associated with vicariant isolation may have originated the stepped clines at appendage size, across the Mondego.

FIG2

Fig. 6. Trend surface map generated by the kriging of mean factor scores of the first Principal Component axis of 18 populations of female Chioglossa lusitanica.

Neither colour pattern nor colour pattern variability was associated with group membership or with geographic distances between populations. However, colour pattern variability was higher within the group 1-2 contact zone than elsewhere, suggesting that the mixing of differentiated gene pools increased phenotypical variability. Two more parallels between morphology and genetics were found within group 2 populations. First, a south to north decrease was observed in genetic and colour pattern variability. The processes of sequential bottlenecking and drift invoked to explain the decrease in genetic variation (Alexandrino et al., 2000) appear equally applicable to morphological variation. Secondly, the dominance of the otherwise rare colour type 3 in populations 10 and 11, immediately south of the Douro, may reflect a separate historical refugium in this area, as was suggested by the local presence of unique nuclear and mtDNA alleles (Alexandrino et al., 2000, 2002).

The genetic subdivision of C. lusitanica is not matched by an equally pronounced morphological differentiation. Selection operating along environmental gradients appears to be more important in shaping phenotypic diversity than genetic isolation. Cryptic differentiation and clines are phenomena common to many amphibian species (Larson, 1984; Green et al., 1996), streamside salamanders in particular (Good and Wake, 1992; Carlin, 1997; Tarkhnishvili et. al., 2000). For example, C lusitanica and Mertensiella caucasica share ancestry at 14-15 million years before present (Veith et al., 1998) with the further intraspecific genetic differentiation (roughly 1-2 and ~10 million years before present, respectively) being accompanied by morphological stasis (Alexandrino et al. 2000, present paper; Tarkhnishvili et al., 2000). The two species show strong interdependences in ecological, morphophysiological, reproductive and developmental features associated to a streamside life-history (Tarkhnishvili, 1994). In combination with the remarkable evolutionary convergence observed among streamside salamanders, such as C. lusitanica and phylogenetically unrelated plethodontid species (e. g., Eurycea longicauda ; Wake and Özeti, 1969), this suggests that constraints are in place that prevent departure from a highly specialized morphotype.