Spermatozoa are Endosymbionts

In recent decades, it has become a consensus that mitochondria correspond to descendants of independent creatures that somehow became embedded within an archaeon, forming an inextricable association [1]. It is also considered that this association represents one of the early stages in the development of eukaryotes, the vast group that encompasses all creatures more complex than a bacterium [2].

All eukaryotes, with a single exception [3], and only they, possess mitochondria, or remnants indicating that their ancestors had them, which clarifies part of the puzzle regarding the formation of this chimeric group. However, other notable peculiarities, such as nucleated cells, mitosis, and meiosis, are characteristics shared by all eukaryotes and solely by them. Sexual reproduction is also exclusive to eukaryotes, and although a few of them do not practice it, they all descend from ancestors who did [4]. Thus, directly or indirectly, every eukaryote originated from sexual reproduction in some ancestral branch of its genealogical tree.

Evolutionarily, sexual reproduction replaced binary fission, a simple and efficient mode of reproduction shared by all prokaryotes.

This substitution poses a puzzle highlighted by three compelling considerations:

1. The efficiency of sexual reproduction is significantly reduced compared to binary fission, resulting in the loss of half of the offspring with each generation due to the production of males [5].
2. This paradox is exacerbated by the fact that, even disregarding this loss, sexual reproduction is much more costly and complex than binary fission.
3. The abstrusity deepens when considering that these modes of reproduction do not correspond to homologous phenomena, but to an analogous alteration that ended up replacing the more efficient mode of reproduction previously used by the ancestors of eukaryotes.

The above considerations raise the following questions: What could have led a creature to exchange an extremely simple and efficient mode of reproduction for one that is much more costly, complex and inefficient? Why did this substitution result from an analogous phenomenon rather than the self-improvement of the existing mode of reproduction?

A clue to the answer, as surprising as the considerations that prompt such inquiries, has already been explicitly stated above: just like mitochondria, spermatozoa are endosymbionts, being the true originators of eukaryotes [6].

According to this conception, spermatozoa originated from predators that, through a long process, gradually became less lethal parasites due to the phenomenon of coevolution between the parasite and the host.

The incorporation of mitochondria allowed creatures belonging to a lineage of archaea to acquire volumes thousands of times larger than those of the beings of that time [7]. Such 'giants' were preyed upon by creatures resembling spermatozoa, like contemporary BALOs (Bdellovibrio and like Organisms) [8]. Over evolutionary time, the coevolution between predator and host gradually softened the relationship between them, until, instead of preying on its host, the attenuated parasite merely injected its genome and mitotic apparatus into it. This allowed the descendants of the original predator to reside for a long time encysted inside their 'giant' hosts, waiting for the right time to hatch and reinfect new creatures.

The extreme intimacy of such coexistence facilitated the surprising phenomenon of retro-parasitism, where the host's genome became incorporated into that of the parasite. From then on, when infecting new hosts, the parasite injected not only the genome of its parasitic ancestors but also the genome of the retro-parasite, thereby becoming a vector for the transmission of its own host. This is the origin of sexual reproduction [6]. The theory provides very elegant answers to the key questions outlined above: the clumsy, complex, and inefficient mode of sexual reproduction is not an improvement of the prokaryotic reproductive mode, binary fission, but rather a coevolved strategy resulting from the arms race between predator and host. Thus, all the oddness caused by the replacement of the reproductive mode is clarified through a natural path between the initial and final states of an evolutionary system.

The theory further asserts that the cell nucleus corresponds to the cyst in which the parasite's genome remained embedded in the host, shedding light on several paradoxical enigmas, such as the emergence of complex structures like nuclear membrane pores and the transport system necessary for the uncoupling of transcription from translation, acquired over the course of the coevolution of these creatures [9]. It also explains the emergence of mitosis and meiosis as a result of the parasite's reproductive process. Additionally, it clarifies an enigmatic yet often underemphasized phenomenon: the odd transition involving the alternation of generations that forces eukaryotes to demonstrate their fitness and survive under two genomic modes, haploid and diploid.

The endosymbiotic theory of the sperm dissolves the numerous set of paradoxes associated with the simultaneous emergence of multiple unique characteristics of eukaryotes from the single assumption that sexual reproduction was not originally a change in the mode of reproduction, but a coevolutionary strategy resulting from interactions between parasite and host.

The unification of the solution to the extensive array of seemingly independent paradoxes concerning the origin of eukaryotes and their peculiar characteristics is precisely what gives strength to the theory.

Having clarified the origin of eukaryotes, further developments in the endosymbiotic theory should shed light on peculiarities related to spermatozoa, such as the surprising number of genes expressed in spermatogenesis, around 2,300, corresponding to approximately one-tenth of the genes in the human genome [10], which can be understood as a vestige of the endosymbiotic origin of the spermatozoon.

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