Studies of algae — suggest eukaryotes “receive many gifts” of bacterial DNA
Nebert, Daniel (nebertdw)
Thu 2/14, 8:04 PM
Horizontal gene transfer (HGT) is a fascinating topic within the purview of gene-environment interactions (i.e. if an organism picks up a foreign gene, then that organism is likely to have an altered response to environmental signals and adversity). HGT is defined as “the movement of genetic information between organisms”; this process includes the spread of antibiotic resistance genes between bacteria, which obviously promotes evolution of pathogens. Many resistance genes evolved, long ago, in natural environments without anthropogenic influence. Today these genes are rapidly spreading to, and among, human pathogens (i.e. this mechanism is contributing to clinical diseases). This is an evolutionary phenomenon, much like the process of tumorigenesis. Bacterial HGT occurs by three well-understood genetic mechanisms [see figure]:
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Transformation = Bacteria take up DNA from their environment.
Conjugation = Bacteria directly transfer genes to another cell.
Transduction = Bacteriophages (bacterial viruses) move genes from one cell to another [This diagram is taken from Evol Med Public Health 2015; 2015: 193–194.]
Once transferred, the genes (most commonly bacterial, viral and fungal DNA) continue to evolve in their new host — often resulting in organisms with greater survival skills. Antibiotic use, in human medicine and agriculture, continually selects for resistant bacteria (e.g. tetracycline and β-lactams commonly fed to animals provide a selective environment for tetracycline and methicillin resistance). After a strain gains resistance by HGT, the bacteria proliferate and continue to evolve, as they move among patients and hospitals. This process has now occurred in many bacterial lineages, resulting in diverse populations of a variety of strains. Ongoing HGT poses a problem for clinical surveillance and treatment. Bacterial populations evolve rapidly (cell division often occurring in 30 minutes) — resulting in diversity that necessitates individual screening to determine effective treatments and to detect new strains — such as methicillin- and high-level vancomycin-resistant Staph aureus (MRSA and VRSA). Even when new drugs and diagnostic tools become available, the persistence of HGT will require ongoing surveillance for newly resistant pathogens, leaving practitioners and researchers “racing against evolution”.
Authors of the 2-page editorial [see attached] expand this topic to algae (green scum; plants having chromosomal pairs, rather than single chromosomes like bacteria do) — found in thermal springs and other extreme environments. It appears that the trasferred genes might help these algae adapt to hostile environments. Even the human genome had been found to contain some microbial genes (but further work showed that such genes found in vertebrate genomes are often contaminants introduced during the process of sequencing). If bacterial genes were continually moving into eukaryotes and being put to use, some have suggested that a pattern of such gene accumulation should be discernible within the eukaryotic family tree. However, no such pattern has been detectable.
The initial sequencing of genomes from two species of red algae (called Cyanidiophyceae) indicates that as much as 6% of their DNA have a prokaryotic (bacterial) origin. These so-called extremophiles — which live in acidic hot springs and even inside rocks — cannot afford to maintain superfluous DNA (i.e. they appear to contain only genes needed for survival). The 13 red algal genomes they studied contain 96 foreign genes, nearly all of them sandwiched between typical algal genes in the DNA fragment sequenced, which makes it highly unlikely they were accidentally introduced in the lab.
The transferred genes seem to transport or detoxify heavy metals, or they help the algae extract nourishment from the environment, or cope with high temperature and other stressful conditions (e.g. salinity, osmolarity). By acquiring genes from extremophile prokaryotes, these red algae have adapted better to increasingly extreme environments. Of course, what’s happening in red algae might not be happening in animals such as humans; humans and all other multicellular eukaryotes, including plants, have specialized reproductive cells — e.g. sperm or eggs or their stem cells — and it would have to be only these types of cells that had picked up foreign DNA that could be passed on.
Science 1 Feb 2o19; 363: 439–440