Deletional bias shapes bacterial genomes - No evolution observed

Deletional bias shapes bacterial genomes - loss of information leads to robust parasites


Excerpt from the abstract: "Although bacteria increase their DNA content through horizontal transfer and gene duplication, their genomes remain small and, in particular, lack nonfunctional sequences. This pattern is most readily explained by a pervasive bias towards higher numbers of deletions than insertions. When selection is not strong enough to maintain them, genes are lost in large deletions or inactivated and subsequently eroded. Gene inactivation and loss are particularly apparent in obligate parasites and symbionts, in which dramatic reductions in genome size can result not from selection to lose DNA, but from decreased selection to maintain gene functionality. Here we discuss the evidence showing that deletional bias is a major force that shapes bacterial genomes."
Mira, A., Ochman, H. & Moran, N.A. Deletional bias and the evolution of bacterial genomes. Trends Genet. 17, 589−596.


Excerpt: "Genome sequences show that parasitic bacteria have 500-1200 genes, free-living bacteria have 1500-7500 genes, and archaea have 1500-2700 genes."


Excerpt: "In contrast, comparative genomic studies have revealed that in some cases, the genomes of bacteria, such as Rickettsia or Mycobacteriaspp. , are reduced. For example, the genomes of Mycobacterium leprae, Yersinia pestis and Salmonella Typhi contain hundreds of degraded genes. The evolution of specialized bacteria, including pathogenic bacteria, consists mainly of gene losses. Moreover, extreme genome decay is often accompanied by a low GC% content. Furthermore, genes that encode “virulence factors” are also found in the genomes of non-pathogenic bacteria, such as free-living bacteria, which may carry more “virulence factors” than do pathogenic bacteria. By counting the number of genes involved in transcription, host-dependent bacteria (including pathogens) were found to have significantly fewer transcriptional regulators than free-living bacteria."


Excerpt: "These results indicate that genome size reduction can occur relatively rapidly and loss of certain genes can speed up the process of bacterial genome compaction."


Excerpt: "What makes the M. leprae genome interesting is its small genomic variance; leprosy genomes are >99.9% similar, and have only 807 polymorphic sites, and only 4–5 subtypes. They have found that ancient leprosy genomes are very similar to modern ones. Thus, due to this similarity if they can replicate factors and conditions that halted the spread of leprosy in the 14th century, then this could possibly put an end to the modern leprosy epidemic.14,15 The leprae genome is very stable,16 and is thus at the end stage of the genome reduction process."

My comment: Horizontal gene transfer (HGT) occurs between bacteria. According to newest research, eukaryotic-to-bacteria HGT can only be demonstrated under optimized laboratory conditions. This means bacteria might get new genetic material only from other bacteria or archaea, that are also experiencing rapid genome degradation. That's why bacteria could never evolve into something other than bacteria. Interesting is that gene loss results in robust parasitic or pathogenic bacteria or highly specialized harmful bacteria. This same phenomenon can be observed all over nature. Loss of biological information leads to negative consequences within organisms. This is far from evolution.