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Inherent levels of somatic chromosomal aberrations among three populations of Sigmodon hispidus from north-central Texas.

ABSTRACT. -- Somatic chromosomal variation in Sigmodon hispidus from three closely situated localities in north-central Texas is restricted to uniformly low levels (mean frequency of abnormal cells = 0.064). Inter- and intrapopulational variation was insignificant, and breaks of chromosomes or of individual chromatids account for most of the observed cytogenetic aberrations. Comparisons of this study with other investigations must be made with caution, however, because of the possibility of more widespread geographic variation in the incidence of chromosomal variation. Further caution is also warranted because of variables inherent to the philosophy of individual investigators. Key words: chromosomal variation; north-central Texas; Sigmodon hispidus.


The hispid cotton rat, Sigmodon hispidus, is a subtropical grasslands mammal which ranges from Panama into the United States as far north as Nebraska (Hall, 1981). The species is practically ubiquitous in Texas, and is usually readily obtainable in sufficient numbers for any number of comparative studies. Its ground-dwelling life style and herbivorous feeding habits make the species an ideal candidate for absorbing residual soil contaminants.

The use of rodents as indicators of mutagenesis resulting from exposure to contaminated environments has not been fully explored (McBee and Bickham, 1990). McBee et al. (1987) reported the cytogenetic effects of petrochemical pollutants on natural populations of small mammals. They described a significantly higher incidence of somatic chromosomal abnormalities among the 10 individuals of S. hispidus from a petrochemical waste disposal site when compared to 11 animals from a nearby and presumably pristine control site.

A possible criticism, as pointed out by McBee et al. (1987), of studies such as theirs is the lack of knowledge of the inherent genetic (somatic) variability of those species being examined, as well as the extent that such levels of variability may differ geographically. These data for most natural populations, including those for S. hispidus, are presently lacking. The purpose of this study was to evaluate inter- and intrapopulational levels of somatic chromosomal variation among three separate populations of the hispid cotton rat from presumably uncontaminated areas. Only by establishing such baseline data for purposes of comparison can we hope to determine the effects of artificially introduced pollutants or contaminants on natural populations.


A total of 25 specimens of Sigmodon hispidus were live-trapped in natural habitats from three different sites in north-central Texas, using aluminum Sherman live traps baited with rolled oats. Sample sizes and a brief description for each site are presented below.

Collecting localities. -- The first site, collected on 3 February 1991 (n=10; 3.0 mi. S, 0.5 mi. W Windthorst, Archer County), is a 24.3 hectare soil bank dominated by mesquite (Prosopis glandulosa) and Johnsongrass (Sorghum halepense). The second locality was collected on 14 February 1991 (n=10; near Wichita Falls Clinic in Wichita Falls, Wichita County), and is about 40 kilometers northeast of the first site, where broomweed (Amphiachyris dracunculoides) and dense stands of mesquite predominate. The third area, collected on 21 February 1991 (n=5; near Lake Wichita in Wichita Falls, Wichita County), is approximately 32 kilometers north of the first site, and about eight kilometers to the southwest of the second locality. Winter wheat fields abut a housing development here, and traps were set along a fencerow dominated by Johnsongrass and various forbs.

Karyotypic analysis. -- Animals were transported to the laboratory, where they were maintained in cages. They were yeast-stressed for periods of one to three days and karyotyped following the protocol of Lee and Elder (1980), as modified by Baker and Qumsiyeh (1988). Depending on the mitotic index, five to 10 slides were prepared from the cell suspensions of each individual. These were non-differentially stained with a Giemsa solution for microscopic examination. Voucher specimens, accompanied by standard field data, are deposited in the Collection of Recent Mammals, Midwestern State University.

Only isolated cells with a discernibly complete diploid complement (2N=52, Fig. 1; Zimmerman, 1970) were included in the analyses. Individual cells were judged "abnormal" if one or more chromosomal aberrations were observed in a given spread, without regard to type of abnormal chromosomal configuration or to number of aberrations per cell. Initial scoring for each locality was accomplished immediately upon completion of processing of the last individual from each site, by examining from 100 to 500 cells per individual. Slides representing each of the 25 specimens were then disguised and coded before blind scoring of a total of 20 complete spreads per individual.

Frequencies of occurrence of abnormal cells were noted for each individual and lumped for each locality in both initial and blind studies.


No distinction was made between types of somatic mutations, because detected chromosomal abnormalities were almost entirely breaks of chromosomes or of individual chromatids (Fig. 1). Rarely were spreads interpreted as exhibiting a translocation, acentric chromosome, or dicentric chromosome.

Initial scoring, by locality, of 4000 cells from the 10 Windthorst site animals, 1000 cells from the 10 Clinic site animals, and 500 cells from the five Lake site animals produced respective abnormal cell frequencies of 0.117, 0.055, and 0.080, but results from Student's t-tests showed no significant differences between individuals from the same locality.

Blind scoring of 20 cells per slide from each of the 25 individuals (Table 1) statistically analyzed by using Student's t-tests revealed no significant differences between the frequency of occurrence of abnormal cells from samples taken at the Windthorst (f=0.065), Clinic (0.065), and Lake (0.060) sites. Individual frequencies of occurrence of abnormal cells from the pooled sample (results from the blind study) ranged from 0.000 (n=3) to 0.150 (n=1).



Of the 550 cells from 11 individuals of Sigmodon hispidus examined by McBee et al. (1987) from their study's control site, 15 were judged abnormal (f=0.027), and 13 of these were breaks of chromosomes or individual chromatids. Comparing this study with McBee et al. (1987) permits certain conclusions to be drawn pertaining to somatic variation in the hispid cotton rat and the conduction of similar studies in the future.

Analysis of the large sample sizes from the initial scoring in this study provides evidence that a certain degree of somatic chromosomal variation is inherent to natural populations of the species, that breaks of the chromosomes and chromatids account for most of this variation, and that the frequency of occurrence of abnormal cells is rather uniform within populations.

Given the similarity of frequencies from each of the localities in the blind study (Table 1), the discrepencies of the frequency with which abnormal cells were recorded by locality in the initial scoring are attributed to such temporal influences as acquisition of experience and adjustments in scoring methodology and philosophy for each scoring run.

Discrepencies between the mean frequency of abnormal cells from the control site of McBee et al. (1987) in Brazos County of southern Texas (0.027) and the three closely situated sites in Wichita and Archer counties of north-central Texas (0.064) may be partly due to geographic variation in the incidence of such abnormalities. However, the initially scored frequency differences among the three north-central Texas localities indicate that experience is influential. Individual scoring techniques doubtless introduce other variables, for some workers will be more discerning, or perhaps less conservative in the assignment of questionable spreads as "abnormal". The comparisons of results from studies by different workers must therefore be made with caution, and the determination of any widespread geographic variation in somatic chromosomal variability in Sigmodon hispidus must await a broader-scoped study by a single investigator.
TABLE 1. Results of blind scoring of frequency of abnormal cells in 25
individuals of Sigmodon hispidus from three collecting localities
described in text. Descriptive statistics are sample size (n), mean
frequency (f), and standard deviation (SD). Differences between
localities are not significant.

Individuals Cells Frequency
 Normal (abnormal)

Windthorst (n=200, f=0.065, SD=0.034)
W1 19 (1) 0.050
W2 18 (2) 0.100
W3 19 (1) 0.050
W4 19 (1) 0.050
W5 20 (0) 0.000
W6 18 (2) 0.100
W7 18 (2) 0.100
W8 19 (1) 0.050
W9 18 (3) 0.100
W10 19 (1) 0.050

Clinic (n=200, f=0.065, SD=0.047)
C1 17 (3) 0.150
C2 19 (1) 0.050
C3 19 (1) 0.050
C4 20 (0) 0.000
C5 18 (2) 0.100
C6 19 (1) 0.050
C7 18 (2) 0.100
C8 20 (0) 0.000
C9 18 (2) 0.100
C10 19 (1) 0.050

Lake (n=100, f=0.060, SD=0.022)
L1 19 (1) 0.050
L2 19 (1) 0.050
L3 19 (1) 0.050
L4 19 (1) 0.050
L5 18 (2) 0.100


This study was accomplished in partial fulfillment of the requirements for the Master of Science degree in biology at Midwestern State University. I thank Frederick B. Stangl, Jr., John V. Grimes, Nancy Scott, members of my graduate committee, and two anonymous reviewers for their input and comments on an earlier draft of this manuscript.


Baker, R. J., and M. B. Qumsiyeh. 1988. Methods in chiropteran mitotic chromosomal studies. Pp. 425-428, in Ecological and behavioral methods for the study of bats (T. H. Kunz, ed.). Smithsonian Institution Press, xxii + 533.

Hall, E. R. 1981. The mammals of North America. John Wiley and Sons, New York, 2:vi + 601-1181 + 90.

Lee, M. R., and F. F. B. Elder. 1980. Yeast stimulation of bone marrow mitosis for cytogenetic investigations. Cytogenet. Cell Genet., 26:36-40.

McBee, K. and J. W. Bickham. 1990. Mammals as bioindicators of environmental toxicity. Current Mammal., 2:37-88.

McBee, K., J. W. Bickham, K. W. Brown, and K. C. Donnelly. 1987. Chromosomal aberrations in native small mammals (Peromyscus leucopus and Sigmodon hispidus) at a petrochemical waste disposal site: I. Standard karyology. Arch. Environ. Contam. and Toxicol., 16:681-688.

Zimmerman, E. G. 1970. Karyology, systematics and chromosomal evolution in the rodent genus Sigmodon. Publ. Mus., Michigan State Univ., 4:385-454.


Department of Biology, Midwestern State University, Wichita Falls, Texas 76308

Present address: Department of Biology, Texas A & M University, College Station, Texas 77843.
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Author:Berend, Sue Ann
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:Aug 1, 1993
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