Anita. Behav., 1991,42, 141-143

SHORT COMMUNICATIONS

DNA fingerprinting: estimating background band-sharing in banner-tailed kangaroo rats

B R I A N K E A N E , P E T E R M. W A S E R , L Y N N E T T E D A N Z L - T A U E R & D E N N I S J. M I N C H E L L A

Department o f Biological Sciences, Purdue University, West Lafayette, IN 47907, U.S.A.

(Received 13 August 1990; initial acceptance 14 September 1990; final aeeeptanee 8 November 1990; MS. number: AS-707)

DNA fingerprinting can be used to determine parentage and perhaps other genetic relationships between individuals in natural populations. Whether DNA fingerprinting has the statistical power to identify parents or establish degrees of relatedness between individuals will depend on the background level of band-sharing. Background band-sharing is the mean probability that a band in one individual's fingerprint is also present in the fingerprint of a randomly chosen individual from the same population (Jeffreys et al. 1985). Accurate estimates of background band-sharing are necess- ary to determine probabilities of misassigning parentage or of misidentifying one category of relative for another (Lynch 1988).

Recently, estimates of background band-sharing have been published for several avian and mam- malian species based on band-sharing among a small (2-15) number of individuals who were assumed to be unrelated (e.g. Jeffreys & Morton 1985; Georges et al. 1988). However, in most cases a DNA 'fingerprinter' faces a dilemma: the related- ness of individuals is unknown, indeed, deter- mining relatedness wilt generally be a primary goal of the investigation. How, therefore, should the investigator decide which individuals should be sampled to estimate background band-sharing? We have examined this question using a natural popu- lation of banner-tailed kangaroo rats, Dipodomys spectabilis. By comparing band-sharing between pairs of individuals separated by distances ranging from 50 m to 5 km, we have assessed the degree to which estimates of background band-sharing are influenced by the subset of individuals used to estimate them.

Banner-tailed kangaroo rats are solitary desert granivores. Male and female banner-tailed kanga- roo rats live separately inside conspicuous mounds

and juveniles can remain in their natal mounds for months after weaning (Jones 1984, 1987). These facts allow us to infer matrilineal relationships for many juveniles by trapping them at mounds of adult females. They also allow us to measure the distance between different animals' residencies. The kangaroo rat population we sampled is situ- ated in Portal, Arizona and has been censused every 2-4 months since July 1979.

Fingerprinting analyses began with the collec- tion of 5-t0-mg muscle biopsies taken from the quadriceps of 34 banner-tailed kangaroo rats between August and November 1988. Biopsies were performed under light metofane anaesthesia and stored immediately at - 80~ The locations of all individuals' home mounds were known; juven- iles were trapped prior to or in the early phases of dispersal. Nuclear DNA was extracted from each muscle sample using a modification of a procedure described by Georges et al. (1988). DNA samples ( ~ 5 gg) were digested with Hae III, electrophoresed on an 0-8% agarose gel at 40 V, and transferred to a nylon filter. Filters were hybridized overnight at 65~ by the addition of Jeffreys' multi-locus DNA probe 33"6. Filters were subjected to auto- radiography for 7-10 days at -80~

We performed four separate analyses to examine band-sharing among different subsets of individ- uals. The first subset consisted of two groups (N= 4; N=5) of adult banner-tailed kangaroo rats separated by 5 km. Within each group, individuals occupied mounds separated by 5(~100m. The second subset consisted of a cohort of 10 juvenile banner-tailed kangaroo rats born during the 1988 breeding season and occupying a cluster of mounds within 650 m of each other. The third analysis com- pared band-sharing among six adults that occupied mounds in the same area as the juveniles. Our last

0003-3472/91/070141 + 03 $03.00/0 �9 1991 The Association for the Study of Animal Behaviour 141

142 Animal Behaviour, 42, 1

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Distance (m) Figure l. The proportion of bands shared (~'+ SE) between pairs of banner-tailed kangaroo rats as a function of the distance separating the mounds they occupy. Sample sizes are indicated to the right of each value. The overall pro- portion of bands shared is 0.28 + 0.05. To ensure that SES were not underestimated because of the lack of independence of some pairwise comparisons, we calculated variances using the method of Lynch (1990).

analysis examined band-sharing among three females and six putative offspring.

We ran DNA samples from individuals within each subset together on a gel. Three of us (P.W., L.D.T., B.K.) independently scored the resulting individual-specific hybridization patterns. We cal- culated the proportion of bands shared by two indi- viduals as twice the number of shared bands divided by the total number of bands in both finger- prints (Lynch 1990). We calculated band-sharing for all pairwise comparisons of individuals on the same gel but did not attempt comparisons across gels. Band-sharing values were based on the means of the three scorers.

The mean proportion of bands shared (+SE) between the two groups of banner-tailed kangaroo rats separated by 5 km was 0.30_+ 0.05 (N= 20 pair- wise comparisons), while that within the two groups was 0.31+0.07 (N= 16). The mean pro- portion of bands shared between the 10 juveniles occupying mounds within 650 m of each other was 0.27 + 0"04 (N= 45) and that for the six adults from the same area was 0.29_+0.08 (N= 15). The mean proportion of bands shared between mothers and their putative young was 0-56 4- 0.14 (N= 6). There was no relationship between the mean proportion of bands shared between two individuals and the distance separating the mounds they occupy (Fig. 1).

The level of background band-sharing (0.27- 0'31) we found is at the lower range of values that have been reported in other mammals (Jeffreys & Morton 1987; Weiss et al. 1988). Based on our values, band-sharing between parents and off- spring should be 0-63 (Lynch 1988), which is similar

to the observed value suggesting that the estimated background band-sharing levels are close to the 'true' level for this population.

The dispersal pattern of banner-tailed kangaroo rats is characterized by natal philopatry and short distance dispersal, creating the potential for population subdivision or isolation by distance (Wright 1943). Between 1979 and 1984, median natal-to-breeding mound distances in our popu- lation were 17m for males and 30m for females (Jones 1987). Such short distance dispersal might be expected to result in local neighbourhoods of high band-sharing. Nevertheless, band-sharing values at 50-100 m were no different from those at 5 km, more than 50 times the median dispersal distance.

These data indicate that background band- sharing values in banner-tailed kangaroo rats are not highly sensitive to the geographical location of the individuals used to estimate them. A separation of one to two times the median dispersal distance between individuals seems to be more than adequate to ensure that individuals do not share an anomalously high proportion of bands. Whether other species are similar to banner-tailed kangaroo rats in this respect remains to be investigaged but these results suggest that one method of estimating background band-sharing in a population is to sample individuals along a transect, avoiding com- parisons between individuals separated by less than twice the median dispersal distance.

We thank Steve Konieczny, Suzanne Long, Walter Piper, Patty Rabenold and Jessica Young. This work was supported by NSF grant BSR- 8818040.

Short Communications 143

R E F E R E N C E S

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Jeffreys, A. J. & Morton, D. B. 1987. DNA fingerprinting of dogs and cats. Anita. Genet., 18, 1-15.

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Jones, W. T. 1987. Dispersal patterns in kangaroo rats (Dipodomys spectabilis). In: Mammalian Dispersal Patterns (Ed. by B. D. Chepko-Sade & Z. T. Halpin), pp. 119-127. Chicago: University of Chicago Press.

Lynch, M. 1988. Estimation of relatedness by DNA fingerprinting. Mol. Biol. Evol., 5, 584-599.

Lynch, M. 1990. The similarity index and DNA fingerprinting. Mol. Biol. Evol., 7, 478-484.

Weiss, M. L., Wilson, V., Chan, C., Turner, T. & Jeffreys, A. J. 1988. Application of DNA fingerprinting probes to old world monkeys. Am. J. Primatol., 16, 73-79.

Wright, S. 1943. Isolation by distance. Genetics, 28, 114-138.