/. Embryol. exp. Morph. Vol. 34, 1, pp. 253-264, 1975 253 Printed in Great Britain The development of animals homozygous for a mutation causing periodic albinism (ap) in Xenopus laevis ByOLGA A. HOPERSKAYA1 From the institute of Developmental Biology, Academy of Sciences of U.S.S.R., Moscow SUMMARY This paper describes the development of a mutant strain associated with periodic albinism (ap) in the clawed toad Xenopus laevis. The most outstanding feature of this mutation is the instability of the albino state. In the course of the development there is a succession of three periods of pigment expression: (1) complete absence of melanin pigment, (2) appearance of melanin in the pigmented epithelium of the eyes and in small quantities in skin melanophores, (3) disappearance of most pigment granules. Repeated spawnings show that the mutant syndrome is inherited as a recessive trait. Possible ways of analysing pigment cell differentia- tion with the use of the mutation described are discussed. INTRODUCTION The value of the pigment-synthesizing system in vertebrates as a model for the study of gene expression is well known. The pattern of pigmentation as well as the quantitative aspects of its manifestation are properties which allow the analysis of gene function and the study of external influences on these functions during development. Coloration in amphibia results from the interrelations of different pigments synthesized in melanophores, xanthophores, erythrophores and iridophores (Bagnara, 1966). Colour mutations in amphibia are relatively rare and they are of great importance for investigations in developmental biology (Markert & Ursprung, 1971). Albinism presents one of the more valuable pigment mutations. It is charac- terized by the lack of melanin synthesis by pigmented cells of two types: (1) pigmented epithelium cells of the eye; (2) melanophores originating from the neural crest from where they migrate all over the body. True albinos do not contain pigmented granules (melanosomes) either in the skin or in pigmented epithelium. Their eyes appear pink because of the colour of blood in small vessels in the iris stroma and choroid coat. In amphibia, albino mutations have been described up to this time in frogs (Rana temporaria and R. pipiens) and in 1 Author's address: Instituteof Developmental Biology Ac. Sci., U.S.S.R., Vavilov Street 26, Moscow 117334, U.S.S.R. 254 O. A. HOPERSKAYA Fig. 1. Adult toads of Xenopus laevis carrying the mutation of periodic albinism, av. the axolotl (Smallcombe, 1949; Browder, 1972; Smith-Gill, Richards & Nace, 1972; Humphrey, 1967). In Xenopus, albino mutations have not yet been reported (Gallien, 1968; Gurdon & Woodland, 1974; Droin, 1974). An advantageous model for study of melanin synthesis is provided by the mutation periodic albinism (ap) which appeared spontaneously in 1972 in the Xenopus laevis colony of the Institute of Developmental Biology in Moscow. The purpose of the present communication is to describe the normal develop- ment of animals carrying this mutation. Special attention will be paid to the development of melanocytes of the pigmented epithelium of eyes as well as to dendritic melanophores developing in skin and in the choroid coat of the eye. The mutation periodic albinism (ap) is characterized by (1) the complete absence of melanin in oocytes and embryos, (2) the appearance of melanin at larval stages and (3) the almost complete disappearance of melanin in metamorphosed animals. OBSERVATIONS Coloration of adult animals 'Albino' adults (Fig. 1) are characterized by a cream colour dorsally and a silvery-white colour ventrally. Immediately after metamorphosis, animals have pink eyes and pinkish skin; the skin becomes somewhat cream in colour as Periodic albinism in Xenopus 255 1 //m Fig. 2. em, Expelled melanosomes; chc, choroid coat; m, melanosomes; ope, out- growths of pigmented epithelium; pe, pigmented epithelium; phr, photoreceptors; r, retina; v, vacuoles. (A-D) Details of pigmented epithelium structure in adult Xenopus laevis of av strain. (A) Thickened pigmented epithelium without pigmented granules. (B) Part of pigmented epithelium with melanin granules and outgrowths in the vicinity of optic nerve. (C) Vacuolization of pigmented epithelium cells. (D) Clumps of the expelled pigmented granules. (E) Structure of pigmented epithelium in normal adult Xenopus laevis. 17 EMB 34 256 O. A. HOPERSKAYA Table 1. Comparative table of data on pigmentation Numbers refer to stage numbers of Nieuwkoop & Faber (1956). Pigmentation Stage of onset Stage of from egg to of pigmenta- appearance of Stage of complete neurula (oocyte- tion in melanophores depigmentation Genotype derived pigment) eyes in skin in skin +/+ Animal pole and 29-30 32 Depigmentation derived structures does not occur are pigmented ap/ap Eggs and embryos 39-40 43 Between 48 and are milk-white 63 according to colour all over if spawning derived from ap/ap mother they grow. The cream colour is determined apparently by the presence of xanthophores. In adults, black melanin is found only in horny claws on three toes. In addition, males may have a small weakly pigmented strip on the inner surface of forelimbs; i.e. on the nuptial pads. Eyes of adult animals are pink, though around the limbus and in the vicinity of optic nerve pigmented epithelium contains a small quantity of melanin granules. In respect to this attribute, animals carrying the ap mutation differ from true albinos (Hum- phrey, 1967) which would have melanin in their horny claws but not in their pigmented epithelium. In aP animals, those cells of the pigmented epithelium which contain melanin are concentrated in the vicinity of the optic nerve. The number of pigmented granules per cell varies from 3-5 to 30-40 (Fig. 2). Melanophores of the choroid coat contain only a few pigmented granules. Neither internal organs nor coelomic linings contain melanin. A remarkable individual variability in the intensity of pigmentation is observed. Thus av 'albinos' cannot be classified as true albinos. Growth rate and changes in the pattern of pigmentation during development After the standard injection of gonadotropic hormone, adult animals lay milk-white eggs. Their appearance differs sharply from that of normal eggs in which the animal hemisphere is dark and the vegetal hemisphere white (Fig. 3). In the wild-type embryo melanin derived from the oocyte remains during embryogenesis, particularly in ectoderm. avjav and wild-type embryos develop at the same rate during early development, but at later stages, aplav embryos develop more slowly. Though the presence of the pigment melanin is not essen- tial for normal development (Balinsky, 1970), its complete absence is correlated with slower growth and development. This was clearly observed in the course of Periodic albinism in Xenopus 257 Fig. 3. (A) Milk-white eggs of a mutant strain av of Xenopus laevis; x 16. (B) Pigmented eggs of wild-type Xenopus laevis; x 16. development of two synchronously obtained spawnings of av\av and wild-type embryos which were allowed to develop in the dark at a constant temperature of + 20 °C. A noticeable difference in the rate of growth was found at about stage 45 (Nieuwkoop & Faber, 1956) when embryos of both spawnings had achieved the same morphological stage, but wild-type embryos had reached a greater length (12 mm, while a'^a" embryos were only 8 mm). At later stages, avjav embryos also develop slower than wild-type ones. Embryos which are the progeny of the cross a"la" $ x + / + J are pigmented like wild-type embryos but develop at the rate typical for white embryos. The first indication of pigmentation appears in avjav embryos of all spawnings at stage 39-40. First of all, pigmented granules appear in the eye rudiments. Up till stage 42 the dorsal part of a'}jav eyes is pigmented much more heavily than the ventral part (Fig. 4). The pigmented epithelium of dP\av embryos does not become uniformly pigmented while in +/+ embryos the pigmented epithelium is uniformly pigmented from as early as stage 38. At stage 43 the eyes ofaplap embryos are becoming heavily pigmented (Fig. 5). At this same stage melano- phores appear but they are only just visible and are not at first dendritic. In the course of further development the form of skin melanophores becomes more complicated. At stage 45-46 the skin melanophores acquire a complex stellate morphology. However, even at the climax of skin pigmentation av\av mutants never reach the same degree of pigmentation as wild-type tadpoles. In the course of further development up to approximately stage 56, the pigmentation of avjav embryos increases, though the rate and the intensity of their melanin synthesis display the individual variability. Stage 49 is characterized by the start of pigmen- tation in the choroid coat in av\av tadpoles, whereas in wild-type tadpoles the choroid coat is already fully loaded with pigment at this stage. At stage 55, the iris o>{av\av mutants is becoming bleached; it looses pigment earlier than the 258 O. A. HOPERSKAYA A 01 mm 01 mm Fig. 4. Successive stages of differentiation of pigmented epithelium in the a? mutant strain in comparison with those of wild-type tadpoles. (A) Pigmented epithelium of mutant ap larvae before the onset of the appearance of pigmented granules. Stages 37-38. (B) Pigmented epithelium of wild-type larvae. Stages 37-38. (C) Pigmented epithelium of tadpole of av strain. Dorsal part of eye is more pig- mented. Stage 42. (D) Pigmented epithelium of a wild-type tadpole of the same stage. Periodic albinism in Xenopus 259 Fig. 5. White larvae at the stage of completely black eyes. Stage 43; x 10. Fig. 6. (A) Bleaching of eyes in larvae of the mutant strain av. Stage 45; x 12. (B) The same stage of wild-type larvae, x 12. Fig. 7. Illustration of variability in the process of depigmentation in different batches of tadpoles of the mutant strain av at stage 57-58.