Nematodes have a number of genomes, whose size differs and ranges between 50Mb and 250Mb. A number of nematodes have been sequenced, but a number are still being sequenced. It has been established that some nematodes have genomes that are as big as those of mammals, for example, the Parascaris univalens. Most nematodes have small genomes, for example the Bursaphelenchus mucronatus genome. The varying size in the genomes of the nematodes has only been estimated for about fifty species of nematodes, which is a small number as compared to the number or nematode species that exist today. Also, notable about nematode genomes is that they are compact and, therefore, make for a good study of the structure and evolution of genomes.
Research has shown that the size of nematode genomes is similar to that of flatworms, insects and annelids. However, the genomes are smaller than those of invertebrates like echinoderms and mollusks. The causes for the variations in size of the nematode genomes are not known, but they have been linked to spontaneous deletions.
Most nematodes contain the haploid chromosome numbers of n=4-12. Over 300 species of nematodes have been studied and studies indicate that nematodes display a lot of variations in karyotypic. Additionally, it has been found that a third of the genes in the sequenced nematode genomes have no recognizable homologues outside their genus. Also noticeable is the fact that there are high rates of gene losses and gains among the nematode genomes. There are numerous examples of gene acquisitions that have been observed through gene transfers. Below are a few notable things about nematode genomes.
- These genomes tend to be compact in nature.
- Variation in the gene composition of nematode genomes is attributed to extensive gain and loss of genes.
- Nematodes acquire their genes through the processes of de novo formation, gene duplication and horizontal gene transfer among others.
- The process of horizontal gene transfer allows nematodes to acquire new physiological features. In other words, after the transfer of genes, the nematodes appear different from what they were originally.
- Nematodes lose their genes through the processes of gene deletion and evolutionary changes. The genes are lost to a point where they cannot be recognized as homologous to genes in other species.
- Only a few nematode genomes have been sequenced so far. The sequenced nematodes contain multigene transcription units and operons, which give rise to a single pre-mRNA. The pre-mRNA is then broken u into single protein coding mRNAs through the processes of trans-splicing and polyadenylation.