Johnston P G, Watson C M, Adams M, Paull D J
Department of Biological Sciences, Macquarie University, Sydney, Australia.
Cytogenet Genome Res. 2002;99(1-4):119-24. doi: 10.1159/000071583.
Cytogenetic studies have shown that bandicoots (family Peramelidae) eliminate one X chromosome in females and the Y chromosome in males from some somatic tissues at different stages during development. The discovery of a polymorphism for X-linked phosphoglycerate kinase (PGK-1) in a population of Isoodon obesulus from Mount Gambier, South Australia, has allowed us to answer a number of long standing questions relating to the parental source of the eliminated X chromosome, X chromosome inactivation and reactivation in somatic and germ cells of female bandicoots. We have found no evidence of paternal PGK-1 allele expression in a wide range of somatic tissues and cell types from known female heterozygotes. We conclude that paternal X chromosome inactivation occurs in bandicoots as in other marsupial groups and that it is the paternally derived X chromosome that is eliminated from some cell types of females. The absence of PGK-1 paternal activity in somatic cells allowed us to examine the state of X chromosome activity in germ cells. Electrophoresis of germ cells from different aged pouch young heterozygotes showed only maternal allele expression in oogonia whereas an additional paternally derived band was observed in pre-dictyate oocytes. We conclude that reactivation of the inactive X chromosome occurs around the onset of meiosis in female bandicoots. As in other mammals, late replication is a common feature of the Y chromosome in male and the inactive X chromosome in female bandicoots. The basis of sex chromosome loss is still not known; however later timing of DNA synthesis is involved. Our finding that the paternally derived X chromosome is eliminated in females suggests that late DNA replication may provide the imprint for paternal X inactivation and the elimination of sex chromosomes in bandicoots.
细胞遗传学研究表明，袋狸（袋狸科）在发育的不同阶段，会从雌性的一些体细胞组织中消除一条X染色体，从雄性的一些体细胞组织中消除Y染色体。在南澳大利亚甘比尔山的肥胖帚尾袋狸种群中发现了X连锁磷酸甘油酸激酶（PGK-1）的多态性，这使我们能够回答一些长期存在的问题，这些问题涉及被消除的X染色体的亲本来源、雌性袋狸体细胞和生殖细胞中的X染色体失活和重新激活。我们在已知的雌性杂合子的多种体细胞组织和细胞类型中，没有发现父本PGK-1等位基因表达的证据。我们得出结论，与其他有袋类动物群体一样，袋狸中发生父本X染色体失活，并且从雌性的某些细胞类型中消除的是父本来源的X染色体。体细胞中缺乏PGK-1父本活性使我们能够研究生殖细胞中X染色体的活性状态。对不同年龄育儿袋幼崽杂合子的生殖细胞进行电泳分析，结果显示卵原细胞中仅表达母本等位基因，而在前期卵母细胞中观察到一条额外的父本来源条带。我们得出结论，雌性袋狸中失活的X染色体在减数分裂开始时左右发生重新激活。与其他哺乳动物一样，晚期复制是雄性袋狸Y染色体和雌性袋狸失活X染色体的共同特征。性染色体丢失的基础仍然未知；然而，DNA合成的较晚时间与之有关。我们发现雌性中父本来源的X染色体被消除，这表明晚期DNA复制可能为袋狸中父本X失活和性染色体消除提供印记。
