Wrigley J M, Graves J A
Department of Genetics and Human Variation, La Trobe University, Bundoora, Australia.
Chromosoma. 1988;96(3):231-47. doi: 10.1007/BF00302363.
The order Monotremata, comprising the platypus and two species of echidna (Australian and Nuigini) is the only extant representative of the mammalian subclass Prototheria, which diverged from subclass Theria (marsupials and placental mammals) 150-200 million years ago. The 2n = 63 male, 64 female karyotype (newly described here) of the Nuigini echidna is almost identical in morphology and G-band pattern to that of the Australian echidna, from which it diverged about a million years ago. The karyotype of the platypus (2n = 52) has several features in common with those of the echidna species; six pairs of large autosomes, many pairs of small (but not micro-) chromosomes, and a series of small unpaired chromosomes which form a multivalent at meiosis. Comparison of the G-band patterns of platypus and echidna autosomes reveals considerable homology. Chromomycin banding demonstrates GC-rich heterochromatin at the centromeres of many platypus and echidna chromosomes, and at the nucleolar organizing regions; some of this heterochromatin C-bands weakly in platypus (but not echidna) spreads. Late replication banding patterns resemble G-banding patterns and confirm the homologies between the species. Striking heteromorphism between chromosomes of some of the large autosomal pairs can be accounted for in the echidna by differences in amount of chromomycin-bright, late replicating heterochromatin. The sex chromosomes in all three species also bear striking homology, despite the difference in sex determination mechanism between platypus (XX/XY) and the echidna species (X1X1X2X2/X1X2Y). The platypus X and echidna X1 each represent about 5.8% of haploid chromosome length, and are G-band identical. Y chromosomes are similar between species, and are largely homologous to the X (or X1).
单孔目动物包括鸭嘴兽和两种针鼹（澳大利亚针鼹和新几内亚针鼹），是哺乳亚纲原兽亚纲唯一现存的代表，该亚纲在1.5亿至2亿年前与兽亚纲（有袋类动物和胎盘哺乳动物）分道扬镳。新几内亚针鼹的核型为2n = 63（雄性），2n = 64（雌性）（此处为新描述），其形态和G带模式与澳大利亚针鼹几乎相同，两者大约在100万年前分化。鸭嘴兽的核型（2n = 52）与针鼹物种有几个共同特征；有六对大的常染色体、许多对小（但不是微小）染色体，以及一系列在减数分裂时形成多价体的小的不成对染色体。鸭嘴兽和针鼹常染色体的G带模式比较显示出相当程度的同源性。染色体霉素带型显示，许多鸭嘴兽和针鼹染色体的着丝粒以及核仁组织区存在富含GC的异染色质；其中一些异染色质在鸭嘴兽（但不是针鼹）的染色体制片中C带较弱。晚复制带型模式与G带模式相似，并证实了物种之间的同源性。在针鼹中，一些大的常染色体对之间的染色体存在显著的异态性，这可以通过染色体霉素明亮、晚复制的异染色质数量差异来解释。尽管鸭嘴兽（XX/XY）和针鼹物种（X1X1X2X2/X1X2Y）的性别决定机制不同，但这三个物种的性染色体也具有显著的同源性。鸭嘴兽的X染色体和针鼹的X1染色体各自约占单倍体染色体长度的5.8%，并且G带相同。物种之间的Y染色体相似，并且在很大程度上与X（或X1）同源。
