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GENETICS OF THE NORWAY RAT BY ROY ROBINSON PERGAMON PRESS OXFORD LONDON EDINBURGH · NEW YORK PARIS · FRANKFURT Pergamon Press Ltd., Headington Hill Hall, Oxford 4 & 5 Fitzroy Square, London W.l Pergamon Press (Scotland) Ltd., 2 & 3 Teviot Place, Edinburgh 1 Pergamon Press Inc., 122 East 55th St., New York 22, N.Y. Pergamon Press GmbH, Kaiserstrasse 75, Frankfurt-am-Main Copyright © 1965 Pergamon Press Ltd. First edition 1965 Library of Congress Catalog Card No. 64-19588 PRINTED IN POLAND PREFACE THE rat is one of the more extensively employed laboratory animals, yet— somewhat surprisingly—hitherto there has not been available an account of the genetics of the animal. It is true, of course, that the rat has often been considered in a general fashion in volumes on animal genetics, and on occasion the treatment has been fairly thorough. Unfortunately, more usually the treatment has tended to be superficial, depending upon the competing interests of the author or the scope of the volume. This state of affairs should not be allowed to continue, and it is the intention of this book to review and to provide a key to the considerable literature. An attempt has been made to include all relevant material published prior to 1962. In a number of fields it has been possible to include some important work which has appeared at a later date. While every endeavour has been made to achieve comprehensiveness only posterity can decide the success of this aim. Each chapter is essentially monographic in tenor, with numerous cross-references to pertinent aspects to be found in others. The research papers cited should lead to a complete bibliography for the subject. While an effort has been made to include the salient features of a piece of research work, it should be recognized that the coverage may not be necessarily exhaustive. Long discussion of techniques or extensive descriptions of non-genetical matters have, perforce, been considered beyond the scope of this book. It is irritating to find items of genetical interest, such as strain differences, in- terspersed in a report, for example, devoted primarily to a pharmaceutical analysis. Such items are extremely difficult to discover and bring to the notice of the mammalian geneticist. The hint may be offered that a separate note in a genetical journal, drawing attention to the strain difference, would not come amiss. It should not be thought that the subject matter of this book is only the concern or the province of the mammalian geneticist—this is far from the true situation. Any person who has to deal with living animals during any part of his research is confronted with genetic variation of one form or another. Hence, it is felt that the extent of the known genetics of the rat should be within the cognizance of all biologists, especially those who employ the rat at some stage of their work. If the reading of this book instils an added awareness of a probable source of variation in experimental studies (as an undesirable source of error or as a welcome extra dimension of analysis, as the case may be!) one of its main purposes will be achieved. If this book encourages an interest in the understanding of the nature of the variation, then this pur- pose will be doubly achieved. ROY ROBINSON vii ACKNOWLEDGMENTS I SHOULD like to record cordial appreciation to the many people who have assisted me in numerous ways during the writing of this book. I am indebted to Drs. R. A. Beatty, W. E. Castle, T. C. Hsu, H. Kalter, D. Krech, S. Makino, G. K. Manna, J. L. McGaugh, J. R. Nichols, R. D. Owen, E. Roberts, W. K. Silvers, J. O. Sines, A. U. Smith, R. M. Stephan and J. Warkany for various forms of assistance kindly accorded to me. I am particularly grateful to Dr. P. L. Broadhurst for his help with many bibliographical matters. I am also obliged to the many other investigators who generously sent me reprints of their published research. I have to thank Mr. A. Coenders for the benefit of his linguistic advice. Finally, my special thanks are due to Miss Hilda M. Perry for her competent secretarial assistance. INTRODUCTION IN the eyes of the layman, the Norway rat is the laboratory animal par excel- lence. In actuality, of course, the rat has to compete with other experimental animals for its place in the laboratory. In this respect, however, the rat is fully capable of holding its own: it adapts easily and quickly to congenial laboratory routines, it is a convenient size for a vast range of experiments, it is docile and completely tractable in experienced hands, it does not require large or expen- sive cages and is renowned for reproductive proliferacy. Along with other laboratory rodents at the turn of the century, the rat was quickly taken up as a useful animal for establishing the general pattern ot heredity. The early studies were largely directed towards the heredity of coat colour but, as the genetic vista broadened, research with the animal began to assume a wider variety. Some idea of the extent of this may be gleaned from a perusal of the table of contents of this monograph. It is perhaps permis- sible at this juncture to ponder upon the likely place of the rat in future genetic research. The animal, of course, will continue to be useful for com- parative rodent genetics. It will be especially valuable for meticulous study of the aetiology of inherited malformations and diseases. The implications of research of this nature for medical science is too obvious and important to be overlooked or neglected. It is probable, too, that the rat will continue to feature prominently in the field of psychogenetics. There are two reasons for this belief: (1) the well-known regard for the rat as a psychological subject and (2) the awareness of the need for information on the genetic aspects of behaviour and intelligence. The genesis of animal behaviour could become as important for psychology and psychiatry as the aetiology of malformations in animals is for somatic medicine. The extensive employment of the rat in many branches of science has promoted the publication of numerous books on the animal. These books have usually treated the animal in relation to one or a group of specific topics. For example, for those seeking to establish a new rat colony, there is the monographic account of Greenman and Durhing (1923) on the elementary aspects of housing and feeding. In this connection, the first chapter of Farris and Griffiths (1949) and the very informative books of Farris (1950) and Worden and Lane-Petter (1957) should be consulted. The question of the importance of a balanced diet for growth and reproduction of the rat has been considered by Russell (1948). This tersely compiled bulletin is a mine of in- formation on the dietary requirements of the species and reviews practically all the work on the subject. There is also the interesting volume of Wiesner and Sheard (1933) on the maternal behaviour. An important general source- book is that of Farris and Griffith (1949), a well-known manual for informa- 1 2 GENETICS OF THE NORWAY RAT tion on the rat and on the usefulness of the animal for research purposes. A handy book of reference is that of Hagemann and Schmidt (1960), in which considerable data has been brought together in the form of tables. Finally, attention must be drawn to the classical work of Donaldson (1924) on the biological form and function of the rat. The value of this early work of con- sultation will probably never entirely cease so long as rats are bred. The genetics of the rat have featured in a number of reviews from time to time. The earliest of these were Lang (1914) and Wright (1917); to be followed by Haldane (1927) and Castle (1947). In successive editions of his book Genetics and Eugenics, which culminated in the fourth edition of 1930, Castle has endeavoured to keep abreast of current research. A cursory discussion of rat heredity may also be found in Castle's book on mammalian genetics which appeared in 1940. Most of these reviews were largely concerned with the inheritance of coat colour and texture. An exception was the interesting but not wholly accurate epitome of Plate (1938); however, this work did contain accounts of many genetic abnormalities. The most complete discussion of the known hereditary malformations of the rat may be found in the small but admirable volume of Grüneberg (1947a). The rat appears to occupy a solid niche in the study of animal psychology. A monumental amount of research is available on the animal and the total is constantly being augmented. Many of the omnibus expository and review volumes of experimental psychology have devoted one or more sections largely to work accomplished with the rat. There are also the interesting accounts of Lashley (1929) and Maier (1939); while special attention must be drawn to the definitive surveys of Munn (1933, 1950). Although these two books have different titles, they may be regarded as successive editions of the same work: which thus provides a complete and succinct summary of published investiga- tions on the rat. Hall (1951a) has commented that psychologists are gradually coming to appreciate the importance of heredity in the interpretation of their findings. The merging of the twin sciences of genetics and experimental psychology may be seen in the coining of a new term to describe research which uses the techniques of each: psychogenetics. The importance of heredity in experimental biology may be put very simply—so simply that its presentation could be overlooked as over-facile. The phenotype of an organism is potentially determined by the genotype formed at the moment of fertilization. Therefore, from one viewpoint, all traits are determined completely by heredity. Alas, two subsequent processes stand between this elementary viewpoint and the final outcome. Firstly, irregularities in the ensuing development of the zygote which, perhaps, can be expressively summed up as ontogenic error; and, secondly, the reaction of the geno- type to the environment to which it is exposed. Both of these processes are mutually interdependent and both are functions of the genotype. Thus, a zygote becomes an individual which is unique since it is improbable that two zygotes will have identical ontogenic errors or be exposed to exactly the same environmental stimuli. Alas again, while these considerations may assist to promote clear thinking they cannot decide the contributions of genotype, INTRODUCTION 3 environment or ontogenic error in the expression of a given character. Only careful experimentation can help in the clarification of this issue. The final upshot is that however much it may be felt that a given character is de- termined by exogenetic factors, the basic fact remains that the genetic com- ponent of the variation may not be inappreciable. This fact is truly recognized in many quarters, and many experiments are frankly organized either to minimize the genetic differences or to exploit these very differences. In this monograph, the evidence for genetic differences in response to various stimuli will be singled out for emphasis. This procedure will be most apparent for those reports which were not primarily concerned with the genetic implica- tions of the results obtained. Nomenclature of Rat Species The zoological nomenclature of the rat remains essentially as set out by H. H. Donaldson (1924). Generically, the designation Rattus has replaced the obsolete terms Mus or Epimys, while at the species level, the name norvegicus has priority over that of decumanus. The passage of time has given the designa- tion of Rattus norvegicus to the Norway rat. The species may often be found in the older literature, however, under one or more of the now obsolete designations. A cognate species to the Norway rat is the black rat. This particular rat is of more than cognitive interest for three reasons. Firstly, because of the close general resemblance between the two species, secondly, since the black rat appears to have preceded the Norway rat in global migration and is in the process of being displaced by the latter and, thirdly, because the black rat displays a geographical variation of colour. The geographical variation of colour has been regarded as sufficient justification for these to be ranked as sub-species. Zoogeographically, this view may have some merit but, genetically, the colour variation scarcely appears to warrant the status of sub-specification. The black rat is also known as the house rat or roof rat, and all of the several varieties are grouped into one species, Rattus rattus. Three primary categories of sub-species are generally recognized. Rattus rattus rattus (black rat) with the dorsum black and venter grey-black; associated with the cooler temperate countries, especially those of Europe. Rattus rattus alexandrinus (Alexandrine rat) with the dorsum brownish-black, venter grey, dorsal and ventral colours merging gradually into each other; associated with Asia minor and North Africa. Rattus rattus frugivorus (roof rat or tree rat) with the dorsum yellow to reddish-brown, venter white or cream, dorsal and ventral colours sharply demarcated along flanks; associated with the Mediterranean region and other warm climates (Hinton, 1920). Some doubt on the propriety of rigid adherence to the sub-speciation of R. rattus has been mooted by Caslick (1956). This criticism is particularly pertinent in those situations where two or more of the sub-species occur together. Many localities in the North American continent appear to support such co-existence. Caslick has given data which showed that rattus and alexandrinus will interbreed and that the typical coloration of the two forms 4 GENETICS OF THE NORWAY RAT was not lost as a result. The normal sub-species coloration was found to occur in the same litter bred either from rattus X alexandrinus or from rattus X rattus. Nicholson and Warner (1953) have shown that a similar inter-alternation existed for alexandrinus and frugivorus on the Pacific island of New Caledonia. Tokuda (1941) has concluded that the Japanese varieties of rattus, despite their variation of coat colour, all belong to the same species. It appears reasonable to propose that the black rat {rattus) differs from the Alexandrine rat {alexandrinus) only in the possession of a dominant gene for extension of black pigmentation. The evidence is conclusive in this respect and may be found in the papers of Morgan (1909) and Feldman (1926). However, the difference between these two species and some races of frugivorus is possibly a little more complex. Yet, in those localities where a frugivorus type cohabits with rattus or alexandrinus, the difference could arise essentially from a mutant allele of agouti. Feldman found that two agouti alleles may exist, (i) agouti white belly and (ii) agouti grey belly. These alleles could account for the difference in belly colour between alexandrinus and frugivorus, while rattus could bear either agouti allele. Symbolically, the genotypes of these varieties would be rattus AWED or AED, alexandrinus AE and frugivorus AWE; where Aw represents the white bellied agouti gene, A the grey bellied agouti, ED the dominant extension of black and E the normal extension of black pigmentation. Rattus norvegicus is less variable in coat colour than R. rattus. The colour may be briefly described as greyish-brown on the dorsum and silvery-grey on the venter, with possibly a tawny tint. Many wild populations are dimorphic for a black variety which is probably homologous genetically with the non-agouti mutant of the laboratory rat. It has been said that this black variety may be confused with the black rat {R. rattus rattus). Hinton {loc. cit.) has surmised that the melanic form of norvegicus may be on the increase and affords a close parallelism with rattus, but there appears to be no direct evidence for this surmisal. However, the observations of Watson (1944) on the incidence of melanics in the dockland of London are of interest in this connexion. An account of Watson's observations may be found in the succeeding chapter (see Table 2). It is advisable that the distinction between the norvegicus and rattus species be based upon morphological characters and not upon coat colour. The coat colour of the two rats is subject to variation, particularly for rattus as described above, and the variation overlaps. The superficial differences between the rats are useful for a rapid diagnosis and may be set out in opposition as follows: norvegicus rattus Larger size, relatively stocky body. Smaller size, relatively slender body. Muzzle blunt. Muzzle pointed. Ear pinnae small, hairy, thick and opaque. Ear pinnae large, relatively hairless and trans- lucent. Tail stout, never as long as combined length Tail slender, often longer than the combined of head and body. length of head and body. Fur soft, with few harsh, bristly hairs. Fur soft, with many harsh, bristly hairs in adults. INTRODUCTION 5 Nipples: number varies from twelve to Nipples: number relatively constant around fourteen. ten. Pads on soles of the feet relatively small. Pads on soles of the feet relatively large. Early History of the Rat The early history of the two rat species has been outlined by Hinton (1919, 1920); and by Donaldson (1924) in some detail. There is no real evidence that the Greeks or Romans were acquainted with the rat and it would seem probable that the rapid multiplication of the animal to become an economic pest is a phenomenon of modern civilization. The first rat to reach the Western world was rattus. This species was thought to be indigenous to the Indian peninsula and the spread westwards began as a consequence of the opening up of trade routes with the East. It was not known to occur in appreciable numbers in Western Europe before the twelfth century. The black rat was carried to the Americas on the ships of the early explorers, probably reaching both the North and South continents almost simultaneously, in the sixteenth century. In North America, the more northerly regions appear to be populated by rattus rattus, while in the warmer southern states, rattus alexandri- nus has become the more common. Rattus norvegicus apparently invaded Europe, again arriving from the East, early in the eighteenth century. It was surmised by the early chroniclers that it may have reached England somewhere between 1728 and 1730. The spread of the Norway rat across Europe was probably very rapid, with reports of their presence in the major capital cities following quickly in succession. Although the invasion of the black rat preceded that of the Norway by several hundred years, the latter soon almost completely displaced and exterminated the former. This may be due to the fact that the Norway rat is the more powerful and pugnacious of the two species; although the progressive disuse of wood in the construction of buildings has been thought to be an important factor in the deline of the black rat. The rattus species now survives only as small isolated colonies in parts of Europe. The Norway rat apparently reached the North American seaboard by about 1775, whence it soon began to supplant the black rat and to take possession of all suitable habitats; mainly in the northern latitudes. The natural home of norvegicus is said to be in the temperate regions of Asia. To be more precise, the actual region may have been wild country between the Caspian Sea and Tobolsk. It was possible that the animal extended as far as Lake Baikal, where a sub-species (Rattus norvegicus primarius) has been recognized. The Norway rat is very active and dominant in temperate climates. In extremely cold areas, the rat can only exist in man-made shelters and, in warm areas (for example, Asia), it is often unable to displace the rattus species. The rattus animal, in general, is a more arboreal rat than norvegicus. Hinton has remarked that at sea it is the principal ship rat as a consequence of its superior climbing powers. It appears almost certain that the Norway rat came into captivity as the albino. Rat-baiting was a popular sport in England and France, as early as 1800, until it was prohibited some seventy years later. In this sport, between 6 GENETICS OF THE NORWAY RAT 100 to 200 recently trapped wild rats were placed into a pit and a trained terrier was let loose among them. The spectators bet on the time required by different terriers to kill the last rat. Large numbers of rats had to be caught and held in readiness for these spectacles. Richter (1954) has written of the existence of records which indicated that albinos were removed from such collections of rats and retained for breeding and exhibition. From these prosaic beginnings it was likely that individuals found their way into laboratories. If the rats had been frequently handled from birth, they would likely be semi-tame and relatively tractable for experimental purposes. According to Richter (1950, 1954) the first paper on the employment of rats in the laboratory was that of Philipeaux (1856) on the effects of adre- nalectomy. Unfortunately, it is not known if these rats came from an established colony. The animals were referred to as albino R. rattus in the original paper but this was probably an error. From this time onwards there were sporadic accounts of rats being used for various experimental work. However, there appeared to be no systematic attempt to found colonies or to arrange for a supply of breeding animals. The first breeding experiments on the rat were those of Crampe (1877-85) with albino and wild rats. Dr. H. H. Don- aldson, who has devoted his whole life to a study of the animal, was first introduced to the rat in 1893 (Conklin 1939). The rat was apparently first employed in psychological research by Steward (1898), Kline (1899) and Small (1900); see Miles (1930) and Warden (1930). Since that time the rat has been utilized for psychological research on an ever expanding scale. An idea of the impressive amount of work on the rat may be found in the two books by Munn (1933, 1950). In the last few decades an increasing amount of attention has been given to the nature of the genetic basic of the complex behaviour of the rat. The initial impetus for this work was an attempt to gain some insight into the temperament of the rat. This field has proved to be involved to the extreme and has demanded a much closer coming together of genetic and psychological techniques. Interspecies Hybridization The various rats discussed in the preceding section are so similar in outward appearance that it has long been thought that the two species should be capable of hybridization. Even if the hybridization did not occur in nature, it may be possible for the hybrid to be secured under artificial conditions. However, these expectations have not been substantiated in practice. Fortunately, reports of alleged hybrids between species of rats do not appear to have reached the proportions of those for the notorious Leporide, the mythological hybrid between the rabbit (Oryctolagus cuniculus) and the hare (Lepus europaeus); see Robinson (1958). Adams (1957) has since shown that rabbit ova fertilized by hare spermatozoa perish at the late morula or early blastocyst stage of development. There was even some doubt whether the hare spermatozoa actually contributed any hereditary material to the rabbit ova or if the ova was merely stimulated into development by the entry of the spermatozoa. The claim has been made by Van Kempen (1899; see Craft 1938) to have

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