SELECTION FOR BLOOD PRESSURE LEVELS IN MICE

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1 SELECTION FOR BLOOD PRESSURE LEVELS IN MICE GUNTHER SCHLAGER Department of Systematics and Ecology, University of Kansas, Lawrence, Kansas Manuscript received September 21, 1973 ABSTRACT Response to two-way selection for systolic blood pressure was immediate and continuous for about eight generations. In the twelfth generation, the High males differed from the Low males by 38 mmhg; the females differed by 39 mmhg. There was little overlap between the two lines and they were statistically significant from each other and from the Random control line. There appeared to be no more additive genetic variance in the eleventh and twelfth generahons. Causes for the cessation of response are explored. This is probably due to a combination of natural selection acting to reduce litter sizes in the Low line, a higher incidence of sudden deaths in the High line, and loss of favorable alleles as both selection lines went through a population bottleneck in the ninth generation.-in the eleventh generation, the selected lines were used to produce F,, F,, and backcross generations. A genetic analysis yielded significant additive and dominance components in the inheritance of systolic blood pressure. need for animal models of human diseases has prompted a good deal of TFe:earch designed to uncover these models. For defects caused by single genes this has entailed searching large animal colonies for specifically detectable anomolies and then, by the appropriate genetic crosses, to produce a pure-bred colony of afflicted animals. For diseases dependent on the expression of many genes, selective breeding of extreme segments of the animal population should, in a number of generations, lead to a colony of animals each with a higher (or lower) value for the trait than the majority of individuals of the original base population. The end result of such a breeding program would depend largely on the number of loci at which alleles are segregating and at which the alleles contributing to the desired direction of the trait are available for fixation. The more plus alleles available at these loci in the base generation, the further the selection response should go. The methods of selection will be highly dependent on the objectives of the program. If the animal model is the sole objective, intense selection pressure with moderate inbreeding would be efficacious. It would be questionable whether or not to carry a simultaneous control line as more space and effort could be devoted to maintaining a larger population to intensify the selection pressure. If genetic information (heritability, number of loci, mode of inheritance) is the objective, two-way selection, a concurrent random-bred control, avoidance of inbreeding and patience may be the answer. Previously reported selection programs in rodents were concerned with the production of an animal model. In the selection program to be described, artificial Genetlcs 76: March, 1974.

2 538 G. SCHLAGER selection for high and low systolic blood pressure levels was carried out in a synthesized mouse population with the primary objective of determining the genetic characteristics of the trait. Response to selection pressure for this trait has varied in a number of different organisms but has been successful in elevating blood pressure levels in chickens, turkeys, rabbits, and rats. There is one reported case where selection for elevated pressure was not successful ( STURTEVANT 1953). Previous attempts to select for low systolic blood pressure are limited to chickens and turkeys (see review by SCHLAGER 1972). Studies of the nature of the inheritance of systolic blood pressure in experimental animals involve the rat (PHELAN 1968; KNUDSEN et az. 1970; TANASE et al. 1970; LOUIS et al. 1969; OKAMOTO et al. 1966) and mouse (SCHLAGER 1968, 1971; SCHLAGER and WEI- BUST 1967; WEIBUST and SCHLAGER 1968). It is generally agreed that more than a single pair of alleles is involved in determining blood pressure levels although as few as two were found to be sufficient to explain the susceptibility to the hypertensive effects of chronic salt ingestion (KNUDSEN et al. 1970). In genetic crosses between strains or selected lines, or between selected lines and controls, the Fl blood pressures tend to be intermediate between the parental strains but some degree of dominance has clso been demonstrated. MATERIALS AND METHODS The base population for the BPI series was derived from an eight-way cross of the inbred lines (LP/J, SJL/J, BALB/cJ, C57BL/6J, 129/J, CBA/J, RF/J, and BDP/J) followed by three generations of random mating. A total of 165 mice were measured in generation zero from which the three lines, High, Low, and Random, were begun. The lines were closed after the first generation. In the earlier generations, 20 matings were established for each line, avoiding double first-cousin and sib-matings. One of the 20 pairs of mice was mated in each of the three lines on each of 20 working days to ensure that the offspring of the three lines would be exposed to comparable environmental differences during post-partum holding and measurement periods. The mice were weaned into holding cages, sexes separated, with no more than three mice to a standard 11 x 5 x 6 stainless steel cage. A combination of family- and individual-merit selection was practiced. The blood pressures were ranked from lowest to highest in the two selection lines and within each sex the mice with the highest blood pressure were selected to propagate the high line and those with the lowest, the low line. However, no more than four individuals from the same family were chosen. Mice for the continuation of the random line were selected without any knowledge of their blood pressures but not with any strict randomization procedure. After the sixth generation, a number of ccmpmmises were introduced into the design due to budgetary considerations. The number of matings were reduced and the restriction on mating sibs and double first-cousins was not strictly enforced. Between the sixth and seventh generations, the mice were moved from Bar Harbor, Maine to Lawrence, Kansas, where they were then reared in plastic disposable cages (10.5 x 8 x 4.5 ). A change in food from Old Guilford to Purina Laboratory Chow was also made at this time. Better control of temperature was introduced during generation 9, where the fluctuations were limited to about a IO range between 70 F and 80 F most of the year. A constant light-cycle of 12 hours light and 12 hours dark was maintained throughout the entire experiment. Systolic blood pressures were measured with a Physiograph IV (Narco-Biosystems, Inc.) by occluding the flow of blood in the tail and detecting the return of pulse distal to the cuff upon deflation. The mice were unanesthetized but restrained in a small holder mounted on a thermostatically controlled warming plate. The temperature of the plate was maintained at 37.5 f 1.OD. The mouse was introduced into the holder, allowed to become accustomed to the restraint for a few minutes and then blood pressure determinations were made at half-minute intervals. The

3 SELECTION FOR BLOOD PRESSURE 539 validity of the method was determined by simultaneoas direct (carotid artery) and indirect measurements under anesthesia ( SCHLAGER and WEIBUST 1967). Five measurements were taken on each of three days. During the earlier generations, these measurements were taken at 100, 125, and 150 days, plus or minus five days. After generation 6, the three days generally fell between 100 and 150 days of age, although a few mice were measured as early as 85 days. A mean blood pressure was calculated for each day and the three means were then averaged to give an overall measure of systolic blood pressure for a mouse. This overall measure was used in all calculations and as the basis for selection. F, and Backcrosses: Four pairs of matings were established from the mice used to propagate selection generation eleven. Two were High females mated to Low males and the other two were the reciprocal crosses. Forty-four F, progeny were produced and these in turn were used to propagate seven F, families and four families in each of the two backcrosses. In all of the backcrosses, the females were F,'s since these would be younger, more vigorous females than those of the reciprocal lines, which were well over 250 days old by this time. Offspring from these crosses were all weaned and measured as described above fw the selection lines. RESULTS AND DISCUSSION Response to selection: The average systolic blood pressure for the High, Low, and Random lines for 12 generations of selection are shown in Figures 1 and 2. The separation between the selected lines occurred immediately and the difference increased with succeeding generations until the 8th to 12th generation. Fluctuations in response are evident in both sexes and these are for the most part parallel in the three lines. These are undcubtedly related to environmental factors, although attempts to quantify these have been unsuccessful. The fluctuations are more marked in the later generations corresponding to the less stringent environmental and procedural controls imposed in the animal facilities at the University of Kansas. The fluctuations both in the earlier and later generations are more pronounced among the females than in the males. The response in terms of divergence between the selected lines was regressed on cumulated selection differential as shown in Figure 3. Realized heritability was higher in the males (14.1%) but not statistically different from that of females (11.LE%). It is evident from the plot that selection pressure was continously exerted on these lines and it was rather constant in its magnitude throughout the study. Selection limits: Response to selection may cease for a variety of reasons. The most common reasons are those associated with the absence of additional additive genetic variation and those associated with physiological limits to a trait with the consequence that natural selection counteracts any artificial selection pressure. Physiological limits per se have probably not been reached in this selection program since much higher blood pressures are tolerated in other mammals. However, limits may be imposed by the method of measurement which may act in the same manner as physiological limits. An evaluation of deaths in the 12th generation revealed a significantly greater mortality among High males than among either the Lows or Randoms. Among High males almost 30% die in the restraining chamber before or during the blood pressure determination. This compares to 10% in Randoms and 14% in Lows. This difference is significant at the P < 0.05 level (xz = 6.61 at 2 degrees of freedom). Among females the percentages are 9% Highs, 0% Randoms, and 11% Lows, which are not statistically different

4 544 G. SCHLAGER 1 MALES ; 120- E Y W!j 110- tn tn W P J 0 I- F 80- v) HIGH RANDOM LOW r 00 I GENERATION FIGURE 1.-Response to selection for systolic blood pressure in male mice FEMALES HIGH RANDOM LOW 04 I. I *... * I GENERATION FIGURE 2.-Response to selection loor systolic blood pressure in female mice.

5 SELECTION FOR BLOOD PRESSURE IO CUMULATED SELECTION DIFFERENTIAL FIGURE 3.-Response to selection regressed on cumulated selection differential. Estimates of heritability were 14.1 % in males and 11.4% in females. from one another. If the males with extreme blood pressure levels are more likely to die while being measured, this would effectively eliminate a sizeable proportion of the upper end of the distribution from selection. Theoretically, selection limits can be predicted by the response in early generations. DEMPSTER (1955) has shown that the total advance made in a selection program should equal 2N times the gain in the first generation, where N is the effective population size. This prediction assumes that the rate of fixation is low and that the genes act additively. If dominance is present, the calculation will result in an underestimation. Previous work has shown that the genes affecting blood pressure in mice act additively in crosses between inbred strains with relatively high and low blood pressures (SCHLAGER 1968; SCHLAGER and WEIBUST 1967) but crosses between these selected lines showed some degree of dominance (see below). The estimate of total selection response based on this calculation is probably an underestimation. The average divergence between the two selected lines was 7 mmhg in the first generation. The harmonic mean of the number of parents during the entire study was 17.6, giving an estimate of total divergence oi 123 mmhg. The actual response was about one-third this prediction. ROBERTSON (1960) has shown that half of the total response should be achieved in no more than 1.4N generations, and that if the half-life is short of this value, then the majority of alleles favorable to the direction of the selection will have been fixed in the population. The response to blood pressure falls far short of the predicted 25 generations using 1.4N. The number of parents used to propagate the next generation varied considerably throughout this study. During the first six generations, 18 to 38 parents were used as compared to 4 to 18 in the remainder of the study. The High line went

6 542 G. SCHLAGER through a narrow population bottleneck in the 9th generation when only two pairs of mice were fertile. In the Low line the smallest number was four pairs in the same generation. The premature cessation of predicted response may be due to the loss of many favorable alleles as the two lines went through these bottlenecks. Comparison of selection results: There are two comparable selection programs in rodents which used systolic blood pressure levels as the basis for selection. SMIRK and HALL (1958) and PHELAN (1968) reported on the development of a New Zealand (NZ) strain of rats with genetic hypertension that was derived from a single pair of rats of the Wistar strain. Six sublines propagated by sibmatings were established by more than 20 generations of selection. During this period the average change in systolic blood pressure in the hypertensive strain was an increase of about 2 mmhg per generation, resulting in a difference of mmhg above the control in the best subline. Heritability was estimated at about 15% for males and 10% for females. This may be a conservative estimate of heritability, as there was an indication that there was a tendency for the blood pressure levels to plateau after about 15 generations in some of the sublines. Hypertension was also common in the Japanese spontaneous hypertension rat (SHR) resulting from a selection program by OKAMOTO and AOKI (1963). Again the strain was propagated by sib-mating from a single pair of Wistar-derived rats with elevated systolic blood pressure. Rapid response to selection occurred during the first four generations and continued selection pressure yielded little further response. Systolic blood pressures exceeding 200 mmhg are common in the SHR and the strain shows almost 100% incidence of hypertension. The difference between the selected line and the controls is about 70 mmhg in males and 60 mmhg in females (OKAMOTO et al. 1966). Realized heritability was about 20% in males and 28% in females. The selection programs and progress in these two rat experiments differed in a number of ways. The rats of the NZ lines were anesthetized during measurements while those of the SHR were not. BU~AG, MCCUBBIN and PAGE (1971) demonstrated that systolic pressure in the unanesthetized rat s tail is always lower by about 30 mmhg than that in the aorta. In anesthetized rats, reports have shown consistently lower pressures ( SHULER, KUPPERMAN and HAMILTON 1944; FREGLY 1963) or identical pressures (FRANGIPANE and APORTI 1969) to those in the carotid artery. The difference in measuring technique of anesthetizing or not would accentuate the diff ereiices found in mean systolic blood pressure. Both the NZ and SHR lines were begun from a single-pair samples from Wistar and Wistar-derived substrains and in both experiments sib-mating was practiced almost exclusively. One would assume that there was only a small sample of alleles available for selection in either oi these two lines, yet response was rapid in the SHR and very gradual in the NZ. Estimates of realized heritability in the NZ lines were about half that of the SHR line. These rats are also very different physiologically ((PHELAN 1968). In our selection program, the mice were unanesthetized as in the SHR program, but the response was gradual as in the NZ program. The selection lines

7 SELECTION FOR BLOOD PRESSURE 543 were derived from a more heterogeneous base population (8-way cross) and double-cousin and sib-matings were avoided wherever possible. Theoretically, more alleles should be available for selection in our mouse experiments and their fixation should be more gradual, yet the response was not as great as in either of the rat experiments. In retrospect, the eight strains used to produce the base population may not have been the ideal genetic pool for selection. These strains were originally chosen because they were different in origin and maximum genetic heterogeneity was the goal in the 8-way cross. Strains with more extreme systolic blood pressures than these eight were later found in a survey of 20 strains ( SCHLAGER, unpublished) and a more genetically heterogeneous base population could have been produced with different samples of strains. Systolic blood pressures of inbred strains at comparable ages are not available but the strains used in the 8-way cross were measured at 8-10 months of age, as were the other strains in the survey. The difference between the highest (BALB/cJ) and lowest (BDP) of the eight strains was 25 mm Hg and the range of all strains measured was 38 mm Hg. However, the selected high line in this experiment was more than 10 mm Hg higher than any of the inbred strains. Litter size: The High line had larger litters than the Low line in all but one generation (Figure 4). This difference between mean number born varied in magnitude but was greater than one mouse in three generations out of the first six. This suggests that there may have been some effect of natural selection against extremely low blood pressures in the Low line acting as a maternal influence on the trait or the smaller litter size may reflect the loss of those embryos with extreme blood pressure. The Random line had intermediate litter sizes during the first six generations after which it tended to have larger litters than either of the selected lines. Body weight: Body weights were taken each time the blood pressure was determined on a mouse. There was a general trend toward decreasing body weights E4il, z3 I I,,,,,,,, GENERATION FIGURE 4.-Mean litter size far the High and LGW lines.

8 544 G. SCHLAGER TABLE 1 Othr trm'ts measured during selection for systolic blood pressure Trait Sex High Random LOW Body weight (9) M 27.3 f 0.5 (49)* (28) 27.7 f 0.4 (50) F (44) (23) 22.3 f 0.4 (43) Pulse rate (bpm) M (42) 592 f 12 (24) 580 f 11 (38) F (42) (22) 596 c 9 (35) Hematocrit (X) M (20) (15) 51.1 f 0.4 (47) F (28) 51.9?r 1.0 (6) 50.2 f 0.3 (56) JGI (unweighted) M 0.32?: 0.03 (15) 0.38?: 0.04 (15) 0.35 f 0.02 (15) F 0.39 F 0.02 (15) (15) (15) JGI (weighted) M (15) (15) (15) F (15) 0.67 f 0.05 (15) (15) * Mean 1 standard error (sample size). in the three lines during the 12 generations, probably due to the inbreeding with passing generations. The Low line was consistently slightly heavier than the High line and both selected lines were lighter than the Random line by generation 12 (see Table l). The difference between selected lines was not significant but both were significantly lighter than the Random (P < 0.01 ). PHELAN (1968) reported that the NZ spontaneous hypertension rat was lighter than the control line starting at four weeks of age in both sexes. This relationship was not as consistent in the SHR males, and SHR females tended to be heavier than controls beyond 15 weeks of age (OKAMOTO et al. 1966). Clearly in the mouse, differences in blood pressure between lines are not associated with body weight at the ages these are being measured. Pulse rate: In the twelfth generation samples of pulse rate were taken from the physiograph tracings. These values are given in Table 1. The pulse rate was higher in the males of the High line than in the Lows, and the Random line was intermediate. An analysis of variance showed that these differences were not significant (F = 2.84, P > 0.05). In females, the Random line had the highest pulse rate; again these differences were not significant (F = 1.53, P > 0.05). One cannot attribute the blood pressure differences to excitability, which certainly would be reflected in higher pulse rates. Hematocrits: An association between hypertension and hematocrit level has been reported in the medical literature. During the early generations of selection, there was no evidence of a striking difference between the selected lines. In the tenth generation, however, significant differences were found among the lines, with the Random line intermediate to the higher levels in the High line and lower levels in the Low lines (Table 1 ). A similar association was previously reported in the A/J and SWR/J inbred strains of mice were elevated hematocrit was found in the SWR/J strain which

9 SELECTION FOR BLOOD PRESSURE 545 had the higher blood pressure (SCHLAGER 1968). In that study, it was concluded that the association was fortuitous in the sense that the SWR/J strain carried genes for both the elevated blood pressure and hematocrit level. That conclusion was based on the absence of a correlation between these two variables in the F, generation of the crosses between the strains. An F, generation from crosses between the High and Low selected lines also lacked a significant correlation coefficient (r = 0.16, P > 0.05). Hematocrit level appears to be a trait influenced by few genes which respond rapidly to selection ( SCHLAGER, unpublished). The higher hematocrits in the High line may have resulted from the chance fixation of alleles for elevated levels when the line went through its population bottleneck in generation 9. However, one could argue that chance fixation of high alleles in the High line simultaneously with low alleles in the Low line is unlikely. Renin granularity: Right kidneys of a sample of 15 mice of each sex and line of the fifth generation were prepared for histological examination by the methods described in an earlier paper (SCHLAGER 1968). Both a weighted and unweighted juxtaglomerular cell index (JGI) were calculated and these results are shown in Table 1. A two-way analysis of variance for sex and lines main effects was performed for each of the two scores. NQ significant difference could be found between sexes or lines in either analysis. JGI was previously found to be independent of blood pressure level in lines selected for JGI (RAPP 1965) and in A/J and SWR/J strains of mice which exhibited large differences in blood pressure ( SCHLAGER 1968). F, and backcrosses: A comparison was made of the selection lines of generation 12 and the F, and segregating generations produced by parental selection lines of generation 11. These were measured during the same time period, thus avoiding any effects of the fluctuations in systolic blood pressures seen throughout the selection program. The analysis of the first degree statistics is summarized in Table 2. The procedures followed in this analysis were presented in detail by MATHER and JINKS (1971). The data were first analyzed in the original scale of measurement. There were no significant deviations from zero in any of the scaling tests, suggesting that an additive-dominance model was adequate. The genetic parameters were then estimated by the least squares technique in which the six equations based on the contributions of each type family to m, the midpoint be- tween the two selected lines, [d], the sum of the average effect of the alleles, and [h], the deviation of the F, from the midparental value as a measure of overall dominance. The six equations were weighted by the reciprocal of the variance of the means and multiplied through by the value of the coefficient of the genetic parameter to yield three simultaneous equations for the three unknowns. These were then solved by matrix inversion to yield the estimates given in Table 2. The adequacy of the additive-dominance model was then further tested by computing the expected values of the six generation means and comparing these to the observed data. A goodness-of-fit test demonstrated an adequate agreement. To further demonstrate that non-allelic or epistatic interactions was absent. an attempt was made to fit a model including the genetic components [i], additive X additive interactions, [i], additive X dominance interactions, and [Z], domi-

10 546 G. SCHLAGER TABLE 2 Genetic parameters for the selected lines, PI, F,, and backcrosses Original scale mean + standard ermr Logarithmic scale mean f standard error Males Generation (sample size) High (49) (14) (21 1 (401 (23) (50) Scaling tests A B C Genetic parameters [:I Chl Females Generation (sample size) High B, F! Scal i ng tests A B C Genetic parameters m [d] Chl 124 f f f f f f f f f f f f f f f f f f f f i f f f f e f f f f f _ f f f f f f f f f f f nance x dominance interactions. These were shown to be not significantly different from zero. The additive-dominance model also assumes that there is no genotype x environment interactions. A crude test for the presence of this interaction is the comparison of the variances of the two parental lines and the F, generation. The homogeneity of these variances was tested by an F,,, test and showed homogeneity of the variances in the females but some heterogeneity in the males (Fmax = 3.21, P < 0.05). Consequently, the analyses and estimations were repeated using a logarithmic scale where both the absence of non-allelic interaction and the absence of genotype x environment interaction were demonstrated. The data, scaling tests, and estimations of the genetic parameters are also shown in Table 2.

11 SELECTION FOR BLOOD PRESSURE L BCL Fl BCH H FIGURE 5.--Segregating F2 L BCL Fl BCH H F2 and non-segregating generation means in a genetic triangle. Regardless of the scale, the results clearly show that both the [d] and [h] components are significantly different from zero. the [h] component is positive, so we can conclude that dominance is present with the alleles for elevated blood pressure being on the average dominant to those for lowering blood pressure. The ratio of [h] to [d] is a measure of the average degree of dominance; this was higher in females (0.85) than in males (0.54). The relationships among these six generations can be seen in Figure 5 where the data are plotted in the logarithmic scale. TANASE et ae. (1970) made genetic crosses between the SHR and each of three inbred strains of rats. The resulting genetic triangles were much flatter than those of our data with the F, systolic blood pressure very near the mid-parental value. The F, blood pressures were consistently lower than the F, in all three crosses, while the backcrosses to the high SHR strain tended to be nearer the F, value than to the SHR. The genetic parameters calculated by TANASE using MATHER S (1949) variances method did show a sizable dominance component, although in the absence of standard errors, it is difficult to assess its significance. The genetic parameters in the mouse cross correspond clearly to what can be readily seen in the graphic representation of the genetic triangle (Figure 5). There is a large additive component and a smaller dominance component, both of which are significant in the male and female data. This differs from the genetic analysis of blood pressure in crosses between inbred strains (SCHLAGER and WEIBUST 1967; SCHLAGER 1968). The F, males in crosses between A/J and BALB/cJ were intermediate to the parental strains and subsequent backcrosses gave intermediate means,between the F, and the parental strains. The female data did show an F, systolic blood pressure near the high parental strain value. In another cross, A/J X SWR/J, the results were similar, with no dominance in

12 ~, 548 G. SCHLAGER the males but some evidence for dominance in the females. Genetic parameters were estimated in the A/J x SWR/J cross and the dominance component was not significant. Of these three strains (A/J, BALB/cJ, and SWR/J), only the BALB/cJ was used in the eight-way cross to produce the base generation for selection. There were evidently alleles contributed by some of the other seven strains which show dominance for elevated systolic blood pressures. I am grateful to DR. THOMAS H. RODERICK for samples of mice from his eight-way cross stocks. This research was supported in part by grant HE from the National Heart Institute, a grant from the Kansas Heart Association, an allocation from the Biomedical Sciences Support Grant RR-07037, and the General Research Fund of the University of Kansas. LITERATURE CITED BURAG, R. D., J. W. MCCUBEIN and I. H. PAGE, 1971 Lack of correlation between direct and indirect measurements of arterial pressure in unanaesthetized rats. Cardiovascular Res. 5: DEMPSTER, E. R., 1955 Genetic models in relation to animal breeding problems. Biometrics 11: FRANGIPANE, G. and F. APURTI, 1969 Improved indirect method for the measurement of systolic blood pressure in the rat. J. Lab. Clinic. Med. 73: FREGLY, M. J., 1963 Factors affecting indirect determination of systolic bbod pressure of rats. J. Lab. Clinic. Med. 62: KNUDSEN, K. D., L. K. DAHL, K. THOMPSON, J. IWAI, M. HEINE and G. LEITL, 1970 Effects of chronic salt ingestion; inheritance of hypertension in the rat. J. Exptl. Med. 132: LOUIS, W. J., R. TABEI, A. SJOERDSMA and S. SPECTOR, 1969 in spontaneously hypertensive rat. Lancet: MATHER, K., 1949 MATHER, K. and J. L. JINKS, 1971 York. Biometrical Genetics. Methuen, London Inheritance of high blood-pressure Biometrical Genetics. Cornel1 University Press, Ithaca, New OKAMOTO, K. and K. AOKI, 1963 Development of a strain of spontaneously hypertensive rats. Japan. Circul. J. 27: OKAMOTO, K., R. TASEI, M. FUKUSHIMA, S. NOSAKA, Y. YAMORI, K. ICHIJIMA, H. HAEBARA, M. MATSUMOTO, T. MARUYAMA, Y. SUZUKI and M. T.~MEGAI, 1966 Further observations of the development of a strain of spontaneously hypertensive rats. Japan. Circul. J. 30: PHELAN, E. L., : RAPP, J. P., : ROBERTSON. A., 1960 The New Zealand strain of rats with genetic hypertension. N. Z. Med. J. Alteration of juxtaglomerular index by selective inbreeding. Endocrinology A theory of limits in artificial selection. Proc. Roy. Soc. B 153: SCHLAGER, G., 1968 Genetic and physiological studies of blood pressure in mice. I. Crosses between A/J, SWR/J, and their hybrids. Can. J. Genet. Cytol. 10: Genetic control of blood pressure by more than one pair of alleles. Proc. Soc. Exptl. Biol. Med. 136: , 1972 Spontaneous hypertension in laboratory animals: A review of the genetic implications. J. Hered. 63: SCHLAGER, G. and R. S. WEIBUST, Genetic control of blood pressure in mice. Genetics 55:

13 SELECTION FOR BLOOD PRESSURE 549 SHULER, R. H., H. S. KUPPERMAN and W. F. HAMILTON, 1944 Comparison of direct and indirect blood pressure measurements in rats. Am. J. Physiol. 141 : SMIRK, F. H. and W. H. HALL, 1958 Inherited hypertension in rats. Nature 182: STURTEVANT, F. M., 1953 Artificial selection program for degenerative diseases in rats. Genetics 38: 696. TANASE, H., Y. SUZUKI, A. OOSHIMA, Y. YAMORI and K. OKAMOTO, 1970 Genetic analysis of blood pressure in spontaneously hypertensive rats. Japan. Circul. J. 34: WEIBUST, R. S. and G. SCHLAGER, 1968 A genetic study of blood pressure, hematocrit and plasma cholesterol in aged mice. Life Sci. 7: Corresponding editor: E. RUSSELL