Biodiversity Science ›› 2016, Vol. 24 ›› Issue (8): 966-976.doi: 10.17520/biods.2016057

• Orginal Article • Previous Article    

How did nucleus and sexual reproduction come into being?

Ping Xie*()   

  1. Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan 430072
  • Received:2016-02-28 Accepted:2016-07-12 Online:2016-09-02
  • Xie Ping

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

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)"

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)"

Fig. 3

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

Fig. 4

Compaction of DNA into chromosome (source: Baidu photo)"

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)"

Fig. 6

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

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)."

Fig. 8

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

Fig. 9

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

Fig. 10

Life cycle of a cyclic parthenogenetic Daphnia (after Ebert, 2005)"

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