细胞核和有性生殖是如何起源的?

doi:10.17520/biods.2016057

摘要/Abstract

摘要：

真核生物的起源是一个根本性的、令人生畏的进化谜题, 目前设想的关于“核”起源的流行情景还远谈不上清晰。关于真核生物的起源可谓众说纷纭, 有共营模型、自演化模型、病毒性真核生物起源模型和外膜假说, 等等。迄今为止, 真核演化的动因则鲜有涉及。笔者发现, 从原核生物到真核生物, 基因组的DNA总量大约增加了3.5个数量级, 而这与现代真核生物的DNA压缩比(packing ratio)惊人地一致! 这样, 仅仅用偶然的吞噬、共生或寄生来解释真核生物的起源, 无论如何是难以让人信服的(其实, 正是内共生理论将人们引入了歧途), 而关键是需要解释基因组为何急剧增大。这可能与DNA的复制错误或多倍化现象不无关系, 当然并非完全排除不同种类个体之间的侧向的基因流动或整合的可能贡献。不难理解, DNA压缩机制的成型应该就是迈向真核生物的关键一步, 自然还伴随了细胞内部的结构分化、更为精巧而复杂的细胞分裂机制的发展, 等等。因此, 本文提出细胞核起源的新学说——压缩与结构化假说。此外, 从分子遗传学的角度来说, “性”一点都不神秘, 就是将两个个体的基因组拼在一起而已, 藉此种族多样的遗传信息分散到了个体之中; 而从生态的角度来看, “性”的原始动机就是与休眠事件的偶联。

关键词: 真核的起源, DNA, 染色体, 压缩比, 压缩与结构化假说, 有性生殖, 休眠事件假说

Abstract

The origin of eukaryote is a fundamental, forbidding evolutionary puzzle, and the popular scenarios of eukaryogenesis are far from being clear. So far, there have been various theories (e.g., syntrophic model, autogenous model, viral eukaryogenesis model, exomembrane hypothesis), but few explain why. I observed that C-value (the amount of DNA contained within a haploid nucleus) increased by 3.5 orders of magnitude from prokaryote to eukaryotes, which is inconceivably close to the packing ratio of DNA in extant eukaryotes. Thus, it is never convincing to explain eukaryogenesis solely by using accident phagocytosis, symbiosis or parasitism (the influential endosymbiont theory unfortunately took the wrong turning!), but what is important is to explain why genome increased so sharply. This may be mainly related to DNA replication errors or polyploidization, of course not completely ruling out the possible contribution from lateral gene flow or genetic integration between individuals of different species. It is above suspicion that successful packing of DNA (finally into chromosome) was a key step towards eukaryogenesis, of course also accompanied with structural differentiation in cell and development of more subtle and complex cell division, and so on. This paper presents “packing and structurization hypothesis” to explain eukaryogenesis. In addition, from a molecular genetic point of view, sexual reproduction is never a mystery as it is just a process to merge two individual genomes, by which diverse genetic information of the species are dispersed into its individuals. On the other hand, from an ecological point of view, the original motivation of “sex” was accidently coupled with dormancy.

Key words: eukaryogenesis, DNA, chromosome, packing ratio, “packing and structurization hypothesis”, sexual reproduction, accident dormancy hypothesis

谢平 (2016) 细胞核和有性生殖是如何起源的?. 生物多样性, 24, 966-976. DOI: 10.17520/biods.2016057.

Ping Xie (2016) How did nucleus and sexual reproduction come into being?. Biodiversity Science, 24, 966-976. DOI: 10.17520/biods.2016057.

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链接本文: https://www.biodiversity-science.net/CN/10.17520/biods.2016057

https://www.biodiversity-science.net/CN/Y2016/V24/I8/966

图/表 10

图1 用蓝色荧光染料染色的海拉细胞(HeLa cells)的核DNA, 中间以及最右的细胞处于细胞间期, 因此整个核被标记, 而左边的细胞正在进入有丝分裂, 因此核DNA已经浓缩(来源: 维基百科)

Fig. 1 Cultured HeLa cells have been stained with Hoechst turning their nuclei blue. The central and rightmost cells are in interphase, so the entire nuclei are labeled. The cell on the left is going through mitosis and its DNA has condensed (source: Wikipedia)

图2 通过冷替代方式固定和准备的Gemmata obscuriglobus(a)和斯氏小梨形菌(Pirellula staleyi)(b)的薄切片透射电镜图。NE: 核膜; N: 拟核; ICM: 细胞质内膜; P: 外室细胞质; PL: pirellulosome (小梨形菌属的一种膜细胞隔室) (引自Fuerst, 2005)

Fig. 2 (a) Transmission electron micrograph of thin section of Gemmata obscuriglobus ﬁxed and prepared via cryosubstitution, showing the membrane-bound nuclear body with its nuclear envelope (NE) surrounding the nucleoid (N) and the more general features of the planctomycete cell plan, including the intracytoplasmic membrane (ICM) and paryphoplasm (P). (b) Transmission electron micrograph of thin section of cryosubstituted cell of Pirellula staleyi displaying compartmentalization into pirellulosome (PI) and P separated by the ICM. N is contained within the pirellulosome and thus compartmentalized and surrounded by the single ICM membrane (cited from Fuerst, 2005)

图3 一种纤毛虫(Nassula ornata)与藻类的共生现象(× 160倍)(来源: Wim van Egmond)

Fig. 3 Symbiosis of the ciliate Nassula ornata with algae (magnification:160 ×) (source: Wim van Egmond)

图4 DNA压缩成染色体的过程(来源: 百度图片)

Fig. 4 Compaction of DNA into chromosome (source: Baidu photo)

图5 根据各类生物的C值推测真核生物DNA的压缩原理, 带箭头的红色虚线表示C值中位数的演化轨迹, 绿色虚线表示大多数真核生物C值的主要分布区间, 问号表示该类生物起源的年代仍然存在争议。C值引自Gregory (2004)。

Fig. 5 Packing principle of eukaryotic DNA based on “C value” of various organisms. The dotted red line with an arrow indicates evolutionary track of the median “C value”, and the dotted green line shows the major range of the “C value” of most eukaryotes. Question marks indicate uncertainity about the dates of their origins. C values of various organisms are cited from Gregory (2004)

图6 原核(上图, 大肠杆菌Escherichia coli)和真核(下图)细胞的分裂比较(引自Campbell & Reece, 2008)

Fig. 6 Comparison of the cell divisions between bacteria (upper figure, the bacterium Escherichia coli) and eukaryotes (lower figure) (cited from Campbell & Reece, 2008)

图7 细菌结合示意图。1: 供体细胞产生性菌毛; 2: 性菌毛接触受体细胞, 使两个细胞连接在一起; 3: 能移动的质体被切开, 一个单股的DNA被转移到受体细胞; 4: 两个细胞质粒重新环化, 合成第二股链, 产生性菌毛, 两个细胞又称为活性的供体(来源: 维基百科)。

Fig. 7 Schematic drawing of bacterial conjugation. Conjugation diagram. 1, Donor cell produces pilus; 2, Pilus attaches to recipient cell and brings the two cells together; 3, The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell; 4, Both cells synthesize a complementary strand to produce a double stranded circular plasmid and also reproduce pili; both cells are now viable donors (cited from Wikipedia).

图8 酵母的生活史。(1)出芽生殖; (2)细胞融合; (3)孢子(引自Wikipedia)

Fig. 8 The yeast cell’s life cycle. (1) Budding; (2) Conjugation; (3) Spore. (cited from Wikipedia)

图9 绿藻—衣藻的生活史(引自Raven et al, 1992)

Fig. 9 Life cycle of a green alga, Chlamydomonas (cited from Raven et al, 1992)

图10 周期性孤雌生殖水蚤的生活周期(仿Ebert, 2005)

Fig. 10 Life cycle of a cyclic parthenogenetic Daphnia (after Ebert, 2005)

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