Evolution predicts that related organisms will share characteristics and/or genes derived from common ancestors [1]. These shared characteristics are known as homologies. Homologous structures may have a slight variation in appearance or function, but they originate from the same ancestor. Figure 1 demonstrates an example of homologous structures found across species on a macroscopic scale; however, homology can be found on a microscopic scale in cells, organelles, proteins and DNA.
Homologs can be further divided into two categories: orthologs and paralogs, depicted in Figure 2. Orthologs are genes in different species that evolved from a common ancestral gene by speciation. Normally, orthologs retain the same function in the course of evolution. Identification of orthologs is critical for reliable prediction of gene function in newly sequenced genomes. On the other hand, paralogs are genes related by duplication within a genome. Orthologs retain the same function in the course of evolution, whereas paralogs evolve new functions, even if these are related to the original one [2].
Gene homology can be detected through the BLAST (Basic Local Alignment Search Tool) and FASTA programs. Comparisons of nucleotide or amino acid sequences between species are utilized to determine statistical significance of similarity [3].
Figure 1. An example of homologous bone structures seen in the fore limb of various species.
Figure 2. Relationship between orthologs and paralogs.
SYNGAP1 Protein Homology
Below are the homologs for the SYNGAP1 protein across species compared to humans.
SYNGAP1 is a regulatory protein guiding synaptic plasticity within neurons. However, a key component of this function includes activating various proteins through GTPase activity which can be found outside of the nervous systems. These proteins can be a part of the RasGAP or simply the GAP protein families depending on which proteins they activate. Thus, we see homologs in organisms despite lacking a nervous system. Additionally, it is important to note that there is increased conservation across species that contain similar nervous systems.