Case StudyXX/XY Blood Lymphocyte Chimerism in Heterosexual Dizygotic Twins from an American Bashkir Curly Horse. Case Report
Introduction
Twin pregnancies are undesirable in horses because of the serious economic loss it entails as a result of the high rate of abortion and a tendency for poor postnatal development in the few foals that survive to term. Previous studies conducted on Thoroughbreds reported twin pregnancies terminating in abortion or stillbirth of both foals at a rate of 64.5% from 3 months gestation to term, 72.6% from 8 months to term, and only 14.5% of these pregnancies resulted in the birth of two live foals.1 A similar study in 15 pony mares reported only one case of birth of live twin foals (7%).2 In another study, only 11% of 130 mares carrying twins produced live foals, of which only 38% produced live foals in the following year.3 Foals resulting from twin pregnancies and who survive to term are usually smaller by size, have low body weights, and have a 42% greater mortality rate in the neonatal period as compared with singleton foals.1 The rate of twinning in Thoroughbreds is estimated at 3.5%.4
Most of the twin embryos in a mare are associated with ovulation of multiple follicles and result in the development of dizygotic twins.5, 6 Naturally occurring monozygotic (identical) twins in horses are extremely rare and only a few cases have been reported till date,7, 8 with three cases of monozygotic twins after a single embryo transfer9 and embryo micromanipulation.10 Even more unusual than twinning are triplets in Thoroughbred mares,5, 11 and only one monozygotic triplet case has been verified by (deoxyribonucleic acid) DNA typing.12
Since the first description of blood group chimerism in human beings,13 work that was based on earlier observations in cattle,14 many other cases have been reported. In biology and medicine, an animal or an individual that has two genetically distinct types of cells is called a chimera. Chimerism is common in the cattle population, where vascular communication between the placentas of heterosexual twins leads to abnormalities in the female calf, which is sterile and called a freemartin calf.15 Freemartinism is rare in species other than cattle, but has been reported in sheep,16, 17 goats,18 pigs,15 and llamas.19
Freemartinism is rare in horses and it has been argued whether this condition occurs in this species at all.15 The limited number of blood typing and chromosome analysis reports on equine heterosexual twins show that most animals have normal male or female chromosome complements,20, 21 and only few have been reported to be chimeric in their blood cell populations.21, 22, 23 In contrast to cattle freemartins, equine blood chimeras are usually phenotypically and reproductively normal.22, 23 This is probably because choriovascular anastomosis in twin equine pregnancies occurs after sexual differentiation in the fetuses.23, 24
The aim of this study was to genetically and cytogenetically analyze blood lymphocyte populations from a pair of heterosexual twins of the American Bashkir Curly breed to determine the presence or absence of chimerism in peripheral blood and hair samples.
Section snippets
Samples and Animals
Hair follicles and peripheral blood samples were obtained from an American Bashkir Curly horse family: an 18-year-old sire (curly coat), 19-year-old dam (curly coat), and their twin offspring—a filly (smooth coat) and a colt (curly coat) born in 2007. Blood was collected in sterile sodium-heparin and EDTA vacutainers (Vacutainer, Becton Dickinson, Franklin Lakes, NJ, USA).
Cell Cultures, Chromosome Preparations, and Chromosome Analysis
Metaphase chromosome preparations were obtained from short-term peripheral blood lymphocyte cultures using our standard
Chromosome Analysis
Blood lymphocytes of both the male and female twins showed the presence of a mixed population of 64,XX and 64,XY cells, with the predominance of XX cells in the female and XY cells in the male twin. In general, both types of cell populations had normal equine karyotype (Fig 1. A–D). C-banding allowed for unambiguous identification of sex chromosomes (Fig. 2 A–B) and showed that 79% of the cells in the male twin were 64,XY and 21% were derived from the twin of the opposite sex. Similarly, 81% of
Discussion
Twinning has since long been recognized as one of the leading causes of pregnancy loss in mares.2, 29 Most of the twin embryos in the mare are associated with ovulation of multiple follicles and result in development of dizygotic twins. Many methods for the diagnosis and management of twin pregnancy in the mare have been proposed.29 Chimerism is a rarely encountered condition in the horse because twin pregnancies are undesirable and are therefore usually terminated. The basis for chimerism is
Acknowledgments
The authors thank Ms. Diane Mitchell, the breeder of the horses examined in this study, for alerting us to the birth of the twins and for allowing the collection of samples for the work reported in this study.
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Cited by (11)
Diagnosis of XX/XY Blood Cell Chimerism at a Low Percentage in Horses
2018, Journal of Equine Veterinary ScienceCitation Excerpt :This can be due to postzygotic fusion of two distinct embryos rather than an early anastomosis between the vascular systems of twins (one of which has then be reabsorbed). In this latter case, in fact, typically no abnormalities of the reproductive organs are observed in either twins because when vascular anastomosis are formed, sexual differentiation is already undergone [5,9,21]. Conversely and in contrast with previously reported cases [11,22,23], the present case shows no derivatives from female reproductive organs while male organs are almost completely developed.
Prevalence of twin foaling and blood chimaerism in purebred Spanish horses
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