POSTNATAL DEVELOPMENT IN PEROMYSCUS MANICULATUS-POLIONOTUS HYBRIDS I. DEVELOPMENTAL LANDMARKS AND LITTER MORTALITY 12

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1 OSTNATAL DEVELOENT IN EROYSCUS ANICULATUS-OLIONOTUS HYBRIDS I. DEVELOENTAL LANDARKS AND LITTER ORTALITY WALLACE D. DAWSON Department Zoology and Entomology, The Ohio State University, Columbus, Ohio ABSTRACT Newborn laboratory hybrids eromyscus maniculatus and. polionotus were observed daily from birth to weaning and compared with fspring each 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. polionotus opened one day later than. maniculatus. Hybrid fspring were less viable than the parent types, with a mortality peak noted during the second postnatal week. revious studies development young mice the North American cricetid genus eromyscus have been conducted by several workers. Svihla (9) surveyed postnatal development in five species, including several races the species. maniculatus. ccabe and Blanchard (9) conducted rather extensive comparative studies the development. maniculatus and two other species. King (98) investigated several aspects development in the. maniculatus subspecies bairdii and gracilis in connection with his studies the behavior in these forms. All these studies have included data on the age appearance various developmental landmarks, such as the time at which the eyes opened or pigmentation the dorsum occurs. However, descriptions the developmental characteristics interspecific hybrids previously have not been available. Likewise, there are no detailed reports postnatal mortality in this genus. In view this deficiency and the widespread interest in genetics and speciation in eromyscus, studies this nature were initiated to explore the effects hybridity upon postnatal development. It was thought that any evidence disruption expected developmental processes or inviability young hybrids might be significance in the origin physiological isolation between closely allied species. STOCKS AND ETHODS eromyscus maniculatus bairdii (prairie deermouse) and. polionotus subgriseus (oldfield mouse) were chosen for this study, since the ability these species to interbreed in captivity to a limited extent was known (Dice, 9; Watson, 9; Liu, 9) and stocks these mice were available at the genetics laboratory The Ohio State University Department Zoology and Entomology, where this study was conducted. Several classes matings were established. These included reciprocal crosses between the parent species, crosses among F x from the female. maniculatus male. polionotus cross, hereafter designated Fi, and reciprocal backcrosses between Fi and the parent species. At least five matings each type were attempted. Daily examinations were made litters from birth to weaning. The age at which pinnae unfolded, at which hair exclusive vibrissae could first be detected upon gross examination, and at which eyes first opened were noted. A daily ^asedupon dissertation submitted in partial fulfillment the degree Doctor hilosophy at The Ohio State University. anuscript received December, 9. resent address: Department Biology, University South Carolina, Columbia, South Carolina. THE OHIO JOURNAL OF SCIENCE (): 8, September, 9.

2 No. OSTNATAL DEVELOENT IN EROYSCUS 9 record was maintained litter mortality and significant features concerning the young mice, such as eczema, striking variations in pelage or state health. Note was also taken newborn litters successfully nursing and any marked parental neglect or defensiveness. Such observations were made on all the hybrid classes matings and upon sample matings each the parental types. In statistical analyses the data, "Students' t" tests were employed to detect any significant differences between means the various classes mice. Comparisons were made between. maniculatus and. polionotus and also between reciprocal backcrosses. Statistical comparisons the Fi or F with either parent type were not performed, since, in general, the means fell between the parental values as expected. Samples fspring from the reciprocal cross, female. polionotus male. maniculatus were too small to be value in statistical comparisons. RESULTS The mean age in days, based upon the means for each litter, at which pinnae evoluted, at which hair first appeared, and at which the eyes first opened are given in Table. innae generally unfolded the second or third day in all types. On TABLE ean age in days appearance developmental landmarks taken by litter Type mating Female ale * -H- innae unfold ean S.E. litters Appearance primary hair ean S.E. litters Eyes open ean S.E. Fi F Fi Fi Fi Fi ±..98±.O.±.O9.±..±.8.±.8.7±..±..±.7 8.±.9.78±..7±..±..9±..±.7.±..±..± ±.8.7±.7.±.9.±..±..±..±..±.±.8 Fi designates progeny from female maniculatus'-mae polionotus. designates. maniculatus. designates. polionotus. the average, pinnae. polionotus projected approximately one day later than. maniculatus a significant difference (<.). The difference between the mean time pinnae unfolding in reciprocal backcrosses Fi to. maniculatus was also significant at the. level, with pinnae unfolding slightly later when. maniculatus was the female in the cross. The mean age pinnae opening in the Fx and F fell between the means the parental types. There was no significant difference between. maniculatus and. polionotus in the mean age appearance first hair and first eyes opening, although the mean age in. polionotus was later than in. maniculatus in both instances. Differences between the reciprocal backcrosses to. maniculatus in the first appearance hair were barely significant at the. level based on total litters. However, this analysis did not take into account the non-independence the litters; thus, the difference is doubtful. The means the Fi hybrids fell between those the parental classes in both the age hair appearance and eyes opening.

3 WALLACE D. DAWSON Vol. In both species and all hybrids, the dorsum was pigmented by the second day, but rarely by the end the first day. A tabulation the absolute and proportional litter mortality by two-day periods is presented in Table. All categories matings exhibited a high initial litter mortality, which is lumped into a pre-five-day period. ost this occurred during the first two days and was the result unattended young, many which were devoured by the parents. Cannibalism was particularly frequent among. polionotus. Young which survived past four days had a high probability TABLE Litter mortality from birth to weaning Upper entry: Absolute mortality Lower entry: roportion mortality Cross Days Age re Fi Fj Fi Fi Oil designates. maniculatus. designates. polionotus. Fi designates progeny from female maniculatusmale polionotus. living to weaning age, although among the Fi much the mortality occurred after five days age. Figure graphically presents the proportional litter mortality. An increase in litter mortality in the Fi is depicted during the second postnatal week. This rise is not apparent in the inter se parental crosses nor in the F. Scattered deaths in several litters different matings accounted for this increase; thus, there is some indication that the mortality is related to hybridity genotype. A chi-square contingency test, grouping the mortality by week and comparing the Fi with the combined progeny the other classes, indicated that the increased death rate Fi during the second week largely accounted for a significant chi-square value (<.). Loss entire litters due to juvenile diarrhea or other causes accounted for certain high mortality values in the backcrosses. ronounced scurf was noted in many young, especially at the age hair appearance. A certain amount scaling is normal in young deermice at this age, but in some it was severe and persisted for several days. Some scurfy young were noted among the fspring all categories matings, but they appeared more commonly in certain matings. Subsequent litters from the same matings sometimes differed in the degree scurfing. However, no correlation between severe 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 the Fi F

4 No. OSTNATAL DEVELOENT IN EROYSCUS matings, one the female. polionotus male F lt and one the female. polionotus male. maniculatus. In the first three instances, more than one young in different litters showed the sores. Such sores appeared about four or five days age and were generally healed by fifteen days age. These lesions did not appear to be bites or injuries from the parents, but had a characteristic appearance not seen elsewhere. FIGURE. roportional litter mortality in parent species and Fi hybrids. DISCUSSION Relatively few the observations in this study are directly comparable to those previous investigations, since in most instances other workers have chosen different races deermice, and these may vary widely from one to another in onset time developmental landmarks. In general, the observation Svihla (9) held true here, that the pinnae unfold after the first day but not later than the third day, following birth in members the subgenus eromyscus, which. maniculatus and. polionotus are members. A more direct comparison can

5 WALLACE D. DAWSON Vol. be made in the case the time at which eyes open, since both Svihla (9) and King (98) made observations on the race. maniculatus bairdii. Svihla found the mean time for eyes opening, based on six litters, to be.7=*=.8 days, while King found the mean age in a sample the same race to be. days. The findings here compare favorably, in that eyes in. m. bairdii opened at a mean age. ±.8 days. Since there was no significant difference in the age appearance the developmental landmarks in the two parent types, except in the case pinnae unfolding, no departure from the parental values in the hybrids would be expected under a multifactoral quantitative genetic model. This generally proved to be the case. Only in the reciprocal backcrosses the Fi to. maniculatus was a significant difference noted in the time pinnae unfolding and perhaps in age pelage appearance. In this case, a maternal factor may have accounted for the difference. The data suggest that Fi mother produces young which develop at a slightly accelerated rate, compared with the progeny from the reciprocal cross. Whether this can be related to reciprocal differences in size inheritance observed in this cross (Dawson, 9) is conjectural. The increased mortality the Fi young during the second postnatal week is sufficient to reduce litter size at weaning to a level below that the parental types, although litter size at birth is intermediate. Factors which affect litter survival may have relevance in physiological isolation between related species, since the hybrid has a reduced probability survival to weaning age. ACKNOWLEDGEENTS The author acknowledges the generous assistance Dr. William B. clntosh who supervised this study. LITERATURE CITED Dawson, W. D. 9. Fertility and size inheritance in a eromyscus species cross. Evolution, 9: -. Dice, L. R. 9. Fertility relationships between some the species and subspecies mice the genus eromyscus. J. ammal., : 98-. King, J. A. 98. aternal behavior and behavioral development in two subspecies eromyscus maniculatus. J. ammal., 9: Liu, T. T. 9. The measurement fertility and its use as an index reproductive isolation among certain laboratory stocks eromyscus. Contrib. Lab. Vert. Biol., 9: -. ccabe, T. T. and Barbara Blanchard. 9. Three species eromyscus. Rood Associates, Santa Barbara. p. Svihla, A. 9. A comparative life history study the mice the genus eromyscus. Univ. ich. isc. ubl. us. Zool., : -9. Watson, argaret L. 9. Hybridization experiments between eromyscus polionotus and. maniculatus. J. ammal., : -.