By: Richard D. Cummings
Genetic evolution can proceed by many pathways, including by ‘vertical inheritance’, the passage of genes from parents to offspring, and horizontally by ‘horizontal gene transfer’ (HGT), which is also termed ‘lateral gene transfer’ (LGT) (1, 2). In humans most all known inheritance is through vertical transfer. But for other organisms there are many such examples of LGT in which a glycogene or related type of gene is transferred from bacteria to a plant or animal (3-5). An early example of this was the discovery of a cellulase gene (encoding a β-1,4-endoglucanase), which is expressed in the esophageal glands of the cyst nematodes Globodera rostochiensis and Heterodera glycines (6). The nematode EGases have 37–44% overall amino acid identity to such genes from various bacteria including B. subtilis. Subsequent more detailed genetic studies have identified acquired cellulase genes in the free-living nematode model organism Pristionchus pacificus (7). These were most likely derived from the cellular slime molds Cavenderia fasciculata and Tieghemostelium lacteum. Another example of this is the discovery of the a glycosyl hydrolase family 31 gene (GH31) derived from Enterococcus bacteria and now found in the genome of the domestic silk moth Bombyx mori (8). The event was thought to occur within the Lepidopteran Order approximately 65-145 million years ago. The essential virulence factor capsular polysaccharides in Vibrio vulnificus also appear to be affected by LGT that created new combinations of glyco-synthetic genes (9).
Recent studies on GH32 glycoside hydrolase genes indicate that in 84 different animal species, ranging from nematodes to crabs, this gene family has been transferred from different types of bacteria by multiple, independent HGT mechanisms (3). Also, for the butterflies of the Papilio genus, recent genetic analyses indicate that in 11 Papilio species there has been 199 HGT acquired genes from 76 microbial species, with Pseudomonadota and Bacillota as common HGT donors across these genomes. Some of the genes include UDP-glucosyltransferase 2 for insect detoxification, and a variety of ABC transporter genes (10). Examples of vertical gene transfer are found in the nodulation genes within the Bradyrhizobium genus which control production of lipo-chitooligosaccharides known as Nod factors (NFs) (11). With regard to the human ABO blood group antigens, of which many are expressed in microbes, there is evidence for so-called bacterial ABO genes being transmitted through both horizontal and vertical gene transmission into two separate groups encoding specific enzymes that have unique specificities (12).
Interestingly, in Archaea it is thought that the acquisition of their glycoside hydrolase (GH) genes may be mostly through LGT, as some archaea do not have GH genes, whereas others have multiple such genes (13). An example is Methanosphaera stadtmanae, which has at least 43 glycosyltransferase genes, but amazingly, it has no GH genes. Another interesting and historically important finding in this area was the discovery that the green microalga Chlorella variabilis NC64A, which acquired an ability to synthesize hyaluronate (HA) from the genes expressed by the virus Paramecium bursaria chlorella virus type 1 (PBCV-1) (14). There is increasing evidence for widespread LGT occurring among organisms in the human microbiome (15). A great example is the LGT between marine to human gut bacteria, where the latter acquired genes encoding enzymes capable of degrading seaweed polysaccharides, which may have coincided with the history of humans beginning to consume seaweed in their diets (16). There is increasing evidence that LGT also occurs in invertebrate and vertebrate genomes (17). But the alternative to LGT is convergent evolution where genes arise independently in organisms to provide similar enzymatic activities; this creates another avenue for expanding the evolutionary repertoire of glycogenes (18).
References
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