Author Institution: Department of Zoology and Entomology, The Ohio State University, Columbus, OhioNewborn laboratory hybrids of Peromyscus maniculatus and P. polionotus were observed daily from birth to weaning and compared with offspring of each of the parent types. The Fi hybrids were intermediate between the parent types in mean age at which pinnae unfolded, hair appeared, and eyes opened. Of the parent types, the pinnae of P. polionotus opened one day later than P. maniculatus. Hybrid offspring were less viable than the parent types, with a mortality peak noted during the second postnatal week

Dawson, Wallace D.

The Ohio Journal of Science

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POSTNATAL DEVELOPMENT IN PEROMYSCUSMANICULATUS-POLIONOTUS HYBRIDSI. DEVELOPMENTAL LANDMARKS ANDLITTER MORTALITY12WALLACE D. DAWSON3Department of Zoology and Entomology, The Ohio State University, Columbus, OhioABSTRACTNewborn laboratory hybrids of Peromyscus maniculatus and P. polionotus were ob-served daily from birth to weaning and compared with offspring of each of the parenttypes. The Fi hybrids were intermediate between the parent types in mean age at whichpinnae unfolded, hair appeared, and eyes opened. Of the parent types, the pinnae of P.polionotus opened one day later than P. maniculatus. Hybrid offspring were less viable thanthe parent types, with a mortality peak noted during the second postnatal week.Previous studies of development of young mice of the North American cricetidgenus Peromyscus have been conducted by several workers. Svihla (1932) surveyedpostnatal development in five species, including several races of the speciesP. maniculatus. McCabe and Blanchard (1950) conducted rather extensivecomparative studies of the development of P. maniculatus and two other species.King (1958) investigated several aspects of development in the P. maniculatussubspecies bairdii and gracilis in connection with his studies of the behavior inthese forms. All of these studies have included data on the age of appearanceof various developmental landmarks, such as the time at which the eyes openedor pigmentation of the dorsum occurs. However, descriptions of the develop-mental characteristics of interspecific hybrids previously have not been available.Likewise, there are no detailed reports of postnatal mortality in this genus. Inview of this deficiency and the widespread interest in genetics and speciation inPeromyscus, studies of this nature were initiated to explore the effects of hybridityupon postnatal development. It was thought that any evidence of disruption ofexpected developmental processes or inviability of young hybrids might be ofsignificance in the origin of physiological isolation between closely allied species.STOCKS AND METHODSPeromyscus maniculatus bairdii (prairie deermouse) and P. polionotus subgriseus(oldfield mouse) were chosen for this study, since the ability of these species tointerbreed in captivity to a limited extent was known (Dice, 1933; Watson, 1942;Liu, 1953) and stocks of these mice were available at the genetics laboratory ofThe Ohio State University Department of Zoology and Entomology, where thisstudy was conducted.Several classes of matings were established. These included reciprocal crossesbetween the parent species, crosses among Fx from the female P. maniculatus Xmale P. polionotus cross, hereafter designated MPFi, and reciprocal backcrossesbetween MPFi and the parent species. At least five matings of each type wereattempted.Daily examinations were made of litters from birth to weaning. The age atwhich pinnae unfolded, at which hair exclusive of vibrissae could first be detectedupon gross examination, and at which eyes first opened were noted. A daily^asedupon dissertation submitted in partial fulfillment of the degree Doctor of Philosophyat The Ohio State University.2Manuscript received December 21, 1964.3Present address: Department of Biology, University of South Carolina, Columbia,South Carolina.THE OHIO JOURNAL OF SCIENCE 66(5): 518, September, 1966.No. 5 POSTNATAL DEVELOPMENT IN PEROMYSCUS 519record was maintained of litter mortality and of significant features concerningthe young mice, such as eczema, striking variations in pelage or state of health.Note was also taken of newborn litters successfully nursing and of any markedparental neglect or defensiveness. Such observations were made on all the hybridclasses of matings and upon sample matings of each of the parental types.In statistical analyses of the data, "Students' t" tests were employed to detectany significant differences between means of the various classes of mice. Com-parisons were made between P. maniculatus and P. polionotus and also betweenreciprocal backcrosses. Statistical comparisons of the MPFi or F2 with eitherparent type were not performed, since, in general, the means fell between theparental values as expected. Samples of offspring from the reciprocal cross,female P. polionotus X male P. maniculatus were too small to be of value in sta-tistical comparisons.RESULTSThe mean age in days, based upon the means for each litter, at which pinnaeevoluted, at which hair first appeared, and at which the eyes first opened are givenin Table 1. Pinnae generally unfolded the second or third day in all types. OnTABLE 1Mean age in days of appearance of developmental landmarks taken by litterTypeFemaleMPMPMPFiMMPFXPMPFimatingXXXXXXXXXMaleMPPMMPFiMPFiMMPFiPNumberof1 * -H-2753544253427510PinnaeunfoldMean S.E.1.88±.142.98±.1O2.61±.O91.25±.252.24±.182.55±.182.07±.132.60±.242.40±.27Numberoflitters255851523222366AppearanceprimaryhairMean4.60±4.78±4.71±4.00±4.39±5.00±4.43±4.50±4.33±S.E..19.12.15.32.22.17.14.43.42Numberoflitters245754325311875Eyes openMean S13.21±13.74±13.35±13.33±13.04±13.23±13.61±14.14±13.20±.E..28.17.19.33.25.22.3040.58MPFi designates progeny from female maniculatus'X-maXe polionotus.M designates P. maniculatus.P designates P. polionotus.the average, pinnae of P. polionotus projected approximately one day later thanP. maniculatus a significant difference (P<.001). The difference between themean time of pinnae unfolding in reciprocal backcrosses of MPFi to P. maniculatuswas also significant at the .001 level, with pinnae unfolding slightly later whenP. maniculatus was the female in the cross. The mean age of pinnae opening inthe MPFx and MPF2 fell between the means of the parental types.There was no significant difference between P. maniculatus and P. polionotusin the mean age of appearance of first hair and first eyes opening, although themean age in P. polionotus was later than in P. maniculatus in both instances.Differences between the reciprocal backcrosses to P. maniculatus in the firstappearance of hair were barely significant at the .05 level based on total litters.However, this analysis did not take into account the non-independence of thelitters; thus, the difference is doubtful. The means of the MPFi hybrids fellbetween those of the parental classes in both the age of hair appearance and eyesopening.520 WALLACE D. DAWSON Vol. 66In both species and all hybrids, the dorsum was pigmented by the second day,but rarely by the end of the first day.A tabulation of the absolute and proportional litter mortality by two-dayperiods is presented in Table 2. All categories of matings exhibited a high initiallitter mortality, which is lumped into a pre-five-day period. Most of this occurredduring the first two days and was the result of unattended young, many of whichwere devoured by the parents. Cannibalism was particularly frequent amongP. polionotus. Young which survived past four days had a high probability ofTABLE 2Litter mortality from birth to weaningUpper entry: Absolute mortalityLower entry: Proportion mortalityCrossMPMPMPFiMMPFjXXXXXXXDaysofAgeMPPMMPFiMPFiMPre-5115.804413.877103.50522.78666.75947.58833.5245-66.04130.0648.0392.0715.0575.0632.0327-83.0216.01314.06901.0114.0506.0959-106.0421.00232.1571.03602.025011-123.0214.00818.0881.0362.0234.0507.11113-142.01410.02115.0742.0713.0342.0252.03215-161.00703.01505.0575.0637.11117-183.0215.0116.02902.0232.0253.04819-203.02104.02001Oil3.0382.03221-221.0072.0041.00502.0236.0751.016M designates P. maniculatus.P designates P. polionotus.MPFi designates progeny from female maniculatusXmale polionotus.living to weaning age, although among the MPFi much of the mortality occurredafter five days of age. Figure 1 graphically presents the proportional littermortality. An increase in litter mortality in the MPFi is depicted during thesecond postnatal week. This rise is not apparent in the inter se parental crossesnor in the MPF2. Scattered deaths in several litters of different matings accountedfor this increase; thus, there is some indication that the mortality is related tohybridity of genotype. A chi-square contingency test, grouping the mortality byweek and comparing the MPFi with the combined progeny of the other classes,indicated that the increased death rate of MPFi during the second week largelyaccounted for a significant chi-square value (P<.001). Loss of entire litters dueto juvenile diarrhea or other causes accounted for certain high mortality valuesin the backcrosses.Pronounced scurf was noted in many young, especially at the age of hairappearance. A certain amount of scaling is normal in young deermice at thisage, but in some it was severe and persisted for several days. Some scurfy youngwere noted among the offspring of all categories of matings, but they appearedmore commonly in certain matings. Subsequent litters from the same matingssometimes differed in the degree of scurfing. However, no correlation betweensevere scurf and mating category was noted.Sores and scabs, especially in the sternal region, appeared on certain young.These sternal lesions were noted in progeny from two of the MPFi X MPFXNo. 5 POSTNATAL DEVELOPMENT IN PEROMYSCUS 521matings, one of the female P. polionotus X male MPFlt and one of the femaleP. polionotus X male P. maniculatus. In the first three instances, more than oneyoung in different litters showed the sores. Such sores appeared about four orfive days of age and were generally healed by fifteen days of age. These lesionsdid not appear to be bites or injuries from the parents, but had a characteristicappearance not seen elsewhere.FIGURE 1. Proportional litter mortality in parent species and Fi hybrids.DISCUSSIONRelatively few of the observations in this study are directly comparable tothose of previous investigations, since in most instances other workers have chosendifferent races of deermice, and these may vary widely from one to another inonset time of developmental landmarks. In general, the observation of Svihla(1932) held true here, that the pinnae unfold after the first day but not later thanthe third day, following birth in members of the subgenus Peromyscus, of whichP. maniculatus and P. polionotus are members. A more direct comparison can522 WALLACE D. DAWSON Vol. 66be made in the case of the time at which eyes open, since both Svihla (1932) andKing (1958) made observations on the race P. maniculatus bairdii. Svihla foundthe mean time for eyes opening, based on six litters, to be 13.67=*=.38 days, whileKing found the mean age in a sample of the same race to be 12.1 days. Thefindings here compare favorably, in that eyes in P. m. bairdii opened at a mean ageof 13.21 ±.28 days.Since there was no significant difference in the age of appearance of the develop-mental landmarks in the two parent types, except in the case of pinnae unfolding,no departure from the parental values in the hybrids would be expected under amultifactoral quantitative genetic model. This generally proved to be the case.Only in the reciprocal backcrosses of the MPFi to P. maniculatus was a significantdifference noted in the time of pinnae unfolding and perhaps in age of pelageappearance. In this case, a maternal factor may have accounted for the difference.The data suggest that MPFi mother produces young which develop at a slightlyaccelerated rate, compared with the progeny from the reciprocal cross. Whetherthis can be related to reciprocal differences in size inheritance observed in thiscross (Dawson, 1965) is conjectural.The increased mortality of the MPFi young during the second postnatal weekis sufficient to reduce litter size at weaning to a level below that of the parentaltypes, although litter size at birth is intermediate. Factors which affect littersurvival may have relevance in physiological isolation between related species,since the hybrid has a reduced probability of survival to weaning age.ACKNOWLEDGEMENTSThe author acknowledges the generous assistance of Dr. William B. Mclntoshwho supervised this study.LITERATURE CITEDDawson, W. D. 1965. Fertility and size inheritance in a Peromyscus species cross. Evolu-tion, 19: 44-55.Dice, L. R. 1933. Fertility relationships between some of the species and subspecies of miceof the genus Peromyscus. J. Mammal., 14: 298-305.King, J. A. 1958. Maternal behavior and behavioral development in two subspecies of Pero-myscus maniculatus. J. Mammal., 39: 177-190.Liu, T. T. 1953. The measurement of fertility and its use as an index of reproductive isola-tion among certain laboratory stocks of Peromyscus. Contrib. Lab. Vert. Biol., 59: 1-12.McCabe, T. T. and Barbara Blanchard. 1950. Three species of Peromyscus. Rood Associates,Santa Barbara. 136 p.Svihla, A. 1932. A comparative life history study of the mice of the genus Peromyscus. Univ.Mich. Misc. Publ. Mus. Zool., 24: 1-39.Watson, Margaret L. 1942. Hybridization experiments between Peromyscus polionotus andP. maniculatus. J. Mammal., 23: 315-316.

Postnatal Development in Peromyscus Maniculatus-Polionotus Hybrids. I, Developmental Landmarks and Litter Mortality

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Postnatal Development in Peromyscus Maniculatus-Polionotus Hybrids. I, Developmental Landmarks and Litter Mortality

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