The Elements of Evolution (41-1) Genetic Inheritance

Genetic Inheritance

Horizontal gene transfer (HGT), a dominant evolutionary process, at least in prokaryotes, appears to be a form of (quasi) Lamarckian inheritance. The rate of HGT and the nature of acquired genes depend on the environment of the recipient organism and, in some cases, the transferred genes confer a selective advantage for growth in that environment, meeting the Lamarckian criteria. ~ Russian-American biologist Eugene Koonin & Russian-American cytologist & geneticist Yuri Wolf

Prokaryotes pick up stray genetic material via transformation (from the environment), horizontal gene transfer, and transduction (viral gift-giving). By contrast, eukaryotes only directly get foreign genes from viruses (transduction).

Eukaryotes do have their own genetic exchange, like the “genetic friends with benefits” of prokaryotic conjugation: intracellular gene transfer from endosymbionts. Eukaryotes arose from fusions of numerous viral and prokaryotic genomes.

The main difference of prokaryotes from eukaryotes is that prokaryotic reproduction is independent of DNA acquisition and recombination. Instead, DNA is obtained from fragmented chromosomes obtained via parasexual means (that is, without reproduction). These mechanisms of DNA exchange are not restricted to gene exchange within species, and therefore traits can and do come from highly divergent organisms. ~ evolutionary microbiologists Thane Papke & Peter Gogarten

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Genes fall into 2 functional classes: informational and operational. Each of these classes have their own evolutionary lineages which are extremely intricate due to regular genetic transfers between organisms.

Informational genes provide the data bank for transcription, translation, and other processes related to conveying genetic information. Operational genes are those involved in cellular housekeeping, such as genes for biosyntheses of amino acids, nucleotides, and lipids, genic regulation, and maintaining cell envelopes.

The 2 gene classes have different inheritance paths. Eukaryotes got almost all of their informational genes from archaeal hyperthermophiles (methanogens), likely one of the earliest life forms. Contrastingly, ~70% of the operational genes of eukaryotes came from bacteria, the ubiquitous prokaryotes. ~40% of those operational genes are from Escherichia: anaerobic, rod-shaped bacteria that now reside in animals’ guts. ~30% are from cyanobacteria, the original photosynthesizers that turned algae into plants.

It is not surprising that nuclear eukaryotic genes are derived from multiple prokaryotic sources. But it is startling that eukaryotic informational genes and operational genes have arisen from different types of prokaryotes.

The coherence of the informational lineage might reflect demanding functional constraints imposed on a tightly integrated set of genes. In contrast, the malleability of the operational lineage might reflect a less demanding functional coupling. ~ American molecular biologist Maria Rivera et al

Complex cell structures result from a genetic stew by copious cooks. The microbiome in a eukaryotic organism take up residence symbiotically and donate genetic bits. Further, viral and bacterial infections also occasionally contribute genetically, albeit with less well-meaning intent.

For macrobes, evolution is invariably a process of coevolution. But then, so it is too for the community of microbes that comprise a microbiome or colony.