Unusual congenital polydactyly in mini-pigs from the breeding group of the Institute of Cytology and Genetics (Novosibirsk, Russia)

The article describes a new phenomenon in the breeding group of mini-pigs at the Institute of Cytology and Genetics (ICG, Novosibirsk): polydactyly (extra digits), which is unusual because the additional digits are situated at the lateral surface of legs or at the lateral and medial ones. This anomaly was first found here in 2017 in adult animals intended for culling due to incorrect positioning of the legs caused by flexor tendon laxity and resulting in weight-bearing on the palmar surface of the proximal phalanges (“bear’s paw”). Therefore, the polydactyly of mini-pigs has a pronounced negative selection effect. A visual survey of the livestock was conducted, and a description of the detected anomaly was compiled. The polydactyly in mini-pigs is a stand-alone trait and is not part of any syndromes. Individuals with polydactyly may have extra digits either on pectoral or on pectoral and pelvic limbs. On thoracic limbs, there may be either one lateral digit or a lateral digit and a medially located rudimentary hooflet. On pelvic limbs, only lateral extra digits can occur. Anatomical and morphological analyses showed that the lateral extra digit is an anatomically complete (“mature”) structure, whereas the medial rudimentary digit consists of only a hooflet without other structures characteristic of normal digits. Cytological examination revealed no specific karyotypic features, except for Robertsonian translocation Rb 16;17 previously reported for the mini-pigs of the same livestock. Cytological findings indicated that the polydactyly and Robertsonian translocation are not linked genetically. Genealogical analysis and results of crosses are consistent with a working hypothesis of recessive inheritance of the trait. Overall, the study shows that this type of polydactyly is anatomically and morphologically unique and not typical of Sus scrofa. In this species, only polydactyly types with medial accessory toes have been described and are usually inherited as a dominant trait with incomplete penetrance. In our case, the results of test crosses indicate recessive inheritance of the trait with varying expression and incomplete penetrance, because of which poorly expressed phenotypes are not visually detectable.

1. Beklemisheva V.R., Perelman P.L., Lemskaya N.A., Kulemzina A.I., Proskuryakova A.A., Burkanov V.N., Graphodatsky A.S. The ancestral carnivore karyotype as substantiated by comparative chromosome painting of three pinnipeds, the walrus, the Steller sea lion and the Baikal seal (Pinnipedia, Carnivora). PloS One. 2016;11(1): e0147647. DOI 10.1371/journal.pone.0147647.

2. Glagolev P.A., Ippolitova V.I. Anatomy of Farm Animals with the Basics of Histology and Embryology. Moscow: Kolos Publ., 1977. (in Russian)

3. Gorbach D., Mote B., Totir L., Fernando R., Rothschild M. Polydactyl inheritance in the pig. J. Hered. 2010;101(4):469-475. DOI 10.1093/jhered/esq037.

4. Graphodatsky A.S., Perelman P.L., O’Brien S.J. Atlas of Mammalian Chromosomes. 2nd ed. N.Y.: Wiley-Blackwell. USA, 2020.

5. Graphodatsky A.S., Radzhabli S.I., Baranov O.K. Chromosomes of Farm and Laboratory Mammals: Atlas. Novosibirsk: Nauka Publ., 1988. (in Russian)

6. Ivanchuk V.A. Biogenetic characteristics of rare and endangered breeds of pigs. Veterinariya Sel’skokhozyaystvennykh Zhivotnykh = Veterinary Medicine of Farm Animals. 2011;2:55-60. (in Russian)

7. Kudryavtsev P.N. Breeding Business in Pig Husbandry. Moscow: OGIZ-Selkhozgiz Publ., 1948. (in Russian)

8. Lakin G.F. Biometrics. Moscow: Vysshaya Shkola Publ., 1990. (in Russian)

9. Lange A., Muller G.B. Polydactyly in development, inheritance, and evolution. Q. Rev. Biol. 2017;92(1):1-38. DOI 10.1086/690841.

10. Lebedev M.I., Zelenevsky N.V. Tutorial on the Anatomy of Farm Animals. St. Petersburg: Agropromizdat Publ., 1995. (in Russian)

11. Malynicz G.L. Complete polydactylism in Papua New Guinea village pig, with otocephalic homozygous monsters. Ann. Genet. Sel. Anim. 1982;14(3):415-420. DOI 10.1186/1297-9686-14-3-415.

12. Nikitin S.V., Knyazev S.P., Shatokhin K.S. Miniature pigs of ICG as a model object for morphogenetic research. Rus. J. Genet. Appl. Res. 2014;4(6):511-522.

13. Ptak W. Polydactyly in wild boar. Acta Theriologica. 1963;6:312-314.

14. Seabright M. A rapid banding technique for human chromosomes. Lancet. 1971;2:971-972. DOI 10.1016/s0140-6736(71)90287-x.

15. Sokolov V.E. Taxonomy of Mammals. Pt. 3. Moscow: Vysshaya Shkola Publ., 1979. (in Russian)

16. Stanyon R., Galleni L. A rapid fibroblast culture technique for high resolution karyotypes. Ital. J. Zool. 1991;58:81-83. DOI 10.1080/11250009109355732.

17. Tickhonov V.N., Kochneva M.L., Bobovich V.E. Introduction of karyotypic polymorphism (2n = 36, 37, 38) in genome of domestic pigs Sus scrofa domestica. Informatcionny Vestnik VOGiS = The Herald of Vavilov Society for Geneticists and Breeding Scientists. 2010;14(4):647-653. (in Russian)

18. Wiesner E., Wheller Z. Veterinärmedizinische Pathogenetik. Jena, 1974. (Russ. ed.: Wiesner E., Wheller Z. Veterinary Pathogenetics. Moscow: Kolos Publ., 1979. (in Russian))