Jablonka E, Lamb M J
Genetics Department, Hebrew University, Jerusalem, Israel.
J Theor Biol. 1988 Jul 8;133(1):23-36. doi: 10.1016/s0022-5193(88)80022-5.
The state of activity and condensation of the sex chromosomes in gametocytes is frequently different from that found in somatic cells. For example, whereas the X chromosomes of XY males are euchromatic and active in somatic cells, they are usually condensed and inactive at the onset of meiosis; in the somatic cells of female mammals, one X chromosome is heterochromatic and inactive, but both X chromosomes are euchromatic and active early in meiosis. In species in which the female is the heterogametic sex (ZZ males and ZW females), the W chromosome, which is often seen as a condensed chromatin body in somatic cells, becomes euchromatic in early oocytes. We describe an hypothesis which can explain these changes in the activity and condensation of sex chromosomes in gametocytes. It is based on the fact that normal chromosome pairing seems to be essential for the survival of sex cells; chromosomal anomalies resulting in incomplete pairing during meiosis usually result in gametogenic loss. We argue that the changes seen in the sex chromosomes reflect the need to avoid pairing failure during meiosis. Pairing normally requires structural and conformational homology of the two chromosomes, but when the regions is avoided when these regions become heterochromatinized. This hypothesis provides an explanation for the changes found in gametocytes both in species with male heterogamety and those with female heterogamety. It also suggests possible reasons for the frequent origin of large supernumerary chromosomes from sex chromosomes, and for the reported lack of dosage compensation in species with female heterogamety.
配子细胞中性染色体的活性和凝聚状态常常与体细胞中的不同。例如，XY雄性个体的X染色体在体细胞中是常染色质且具有活性，但在减数分裂开始时通常会凝聚且无活性；在雌性哺乳动物的体细胞中，一条X染色体是异染色质且无活性，但在减数分裂早期两条X染色体都是常染色质且具有活性。在雌性为异配性别的物种（ZZ雄性和ZW雌性）中，W染色体在体细胞中常被视为一个凝聚的染色质体，在早期卵母细胞中会变成常染色质。我们描述了一个假说，它可以解释配子细胞中性染色体活性和凝聚状态的这些变化。它基于这样一个事实，即正常的染色体配对似乎对性细胞的存活至关重要；减数分裂期间导致配对不完全的染色体异常通常会导致配子发生损失。我们认为性染色体中所见的变化反映了避免减数分裂期间配对失败的必要性。正常配对通常需要两条染色体的结构和构象同源性，但当这些区域变成异染色质时，配对就会避免。这个假说为雄性异配性和雌性异配性物种的配子细胞中发现的变化提供了解释。它还暗示了性染色体频繁产生大量超数染色体的可能原因，以及报道的雌性异配性物种中缺乏剂量补偿的可能原因。
