Geographic Variation in Whole Venom Profiles from the Mottled Rock Rattlesnake (Crotalus lepidus lepidus) in Texas

Abstract

-Venom variation in the mottled rock rattlesnake (Crotalus lepids lepidus) was assessed by polyacrylamide gel electrophoresis. Whole venom samples were stained for general protein, the banding recorded, and these profiles were used to compare the variation within and between populations in Texas. Analyses using agglomerative methods (phenetic clustering methods and neighbor-joining analyses) and ordination from Principal Components analyses provide several geographically correlated groups of rattlesnake venom profiles. Evidence is provided for differences in venom profiles of geographically distinct populations. Venom profiles of C. lepidus collected along the Rio Grande River, though geographically disparate, are generally homogeneous. Thus, the analyses support previously hypothesized dispersal of C. lepidus in Texas along these Rio Grande canyons. A potential region of population interchange between eastern plateau populations and those of the west Texas mountain ranges was identified at the southern end of the Davis Mountains in Brewster Co. Lethality assessments using LD_ analyses reveal variation in venom toxicity to range from 2.20-0.72 mg/kg across the geographic range for this subspecies. Four subspecies of Crotalus lepidus (rock rattlesnakes) are currently recognized; C. 1. lepidus, C. 1. klauberi, C. 1. maculosus, and C. 1. morulus (Campbell and Lamar, 1989). The first two subspecies occur in the United States and adjacent Mexico, whereas the latter two are found only in northern Mexico (Campbell and Lamar, 1989). In the western United States, Crotalus lepidus klauberi, is found in southwest Arizona east to extreme southeast New Mexico. The other U.S. subspecies, C. 1. lepidus, reaches as far east as the Edwards Plateau in central Texas and is believed to intergrade with C. 1. klauberi in extreme West Texas (Klauber, 1972). Little is known about rock rattlesnake populations in general and very little information exists for the Mexican subspecies. The U.S. subspecies are known for their exceptional phenotypic variability (Gloyd, 1936; Klauber, 1972; Vincent, Present Address: Center for Environmental Research and Conservation, 10th Floor Schermerhorn, Mail Code 5557, Columbia University, New York, New York. 10027, USA. E-mail: mrf20@Columbia.edu 2 Nature Conservancy, Talahassee, Florida. * To whom all correspondence should be addressed. r subspecies of Crota us lepidus ( ock rattleakes) are currently recognized; C. 1. lepidus, C. . lauberi, C. 1. maculosus, and C. 1. morulus a pbell and Lamar, 1989). The first two subecies o cur in the United St tes and adjacent xico, whereas the latter two are found only orthern Mexico (Campbell and Lamar, 9). In the western United States, Crotalus leps klauberi, is found in southwest Arizona east 1982). Indeed, both Gloyd (1936) and Klauber (1972) attributed the high intraspecific variation to the discontinuity of habitat, which has led to the isolation of breeding populations. This varia ion in dorsal pattern and color is one of the main taxonomic criteria used for differentiating the subspecies (Campbell and Lamar, 1989). Dorsal patterning varies from bands to spots wi h further variation in the number of those marks, and more generally, in the amount of total patterning. In addition, the ground color varies through gray, green, and brown to shades of red or pink. Within the C. I. lepidus populations in Texas, enough variation exists to allow recognition of two distinct (see Fig. 1 in Vincent [1982]). Vincent (1982) used race as a group of regional populations which differ in one or several characteristics from those populations in another region, but he did not propose the elevation of their taxonomic status. Vincent's races are positively correlated to regionally based substrate type. The eastern color form is found in the limestone outcrops of the Edwards and Stockton plateaus, while the more disruptively colored western form is found in the igneous flows of far west Texas. 2). Inde d, both Gloyd (1936) and Klauber 2) at ributed the high intraspecifi variation the discontinuity of habitat, which has led to i olation of breeding populations. This varii n in dorsal pattern and color is one of the in taxonomic riteria used for differentiating subspecies (Campbell and Lam r, 1989). sal at erning varies from bands to sp t 277 This content downloaded from 207.46.13.149 on Mon, 03 Oct 2016 06:10:11 UTC All use subject to http://about.jstor.org/terms M. R. J. FORSTNER ET AL. -~ ,P/o' ~...~,l '' ~4-1 4