PROCEDIMIENTO A SEGUIR PARA LA ACTUALIZACIÓN DE LA BD

Analysing odorant receptor genes in both fish and mammals has revealed that receptors from both groups share some sequence motifs but show only moderate sequence conservation overall and are thought to form separate, non­ overlapping receptor families. This distinction is also reflected in gene-family size and diversity. As many as 1000 different receptors exist in mammals (Buck & Axel, 1991), while the receptor repertoire in fish is thought to be reduced by a factor of ten (Ngai, et a!., 1993). Whether the structure and numerical differences between the two classes of olfactory receptor simply mirrors the phylogenetic distance between fish and mammals or is indicative of adaptive processes allowing fish to detect a limited array of water-soluble aquatic odours while their terrestrial counterparts exploit a more diverse spectrum of hydrophobic airborne odours is still unclear. However, evidence is mounting for the adaptive nature of olfaction in the different environments.

Freitag, at al. (1998), compared the olfactory receptor sequences isolated from aquatic and semiaquatic species representing different levels of vertebrate evolution: amphibia {Rana esculenta and Xenopus laevis), fish {Carassius auratus

and Latimeria chalumnae) and the striped dolphin {Stenella coeruleoalba).

Fish, it seems, possess only Class I genes, whereas mammals are endowed selectively with Class II genes. Fish-like and mammalian-like receptors isolated from the amphibia were found to be expressed in two different compartments of the animals’ nose, which are thought to be employed in the selective recognition of either water-soluble or air-borne odourants. Sequence analysis of amphibian ORs showed that both classes of receptors share a common secondary structure and share several highly-conserved amino acid residues, indicating that both classes originated from common ancestral genes. It is therefore conceivable that both classes may be specialised for recognising the distinct types of odorous ligands in the respective environment; class I for water-soluble, class II for airborne odourants.

It was also found that the coelacanth, Latimeria chalumnae, a species that has been described as one of the closest living relatives to the tetrapods (Betz, et al., 1994), possesses both classes of receptor genes. However, in this “living fossil" most of the class II receptor genes represent non-functional pseudogenes, while no Class I pseudogene has yet been found. Hughes (1993) suggested that the presence of pseudogenes indicates the absence of selective pressure, therefore in

Latimeria, class II receptors may not be of functional importance. Assuming that an ancient lobe-finned relative of the coelacanth was adapted to semi-aquatic life, this “loss of function” of class II genes may indicate a secondary adaptation to return to a totally aquatic lifestyle. Similarly, marine mammals have undergone a secondary

transition to a fully aquatic existence, having evolved from terrestrial ancestors (Carroll, 1988). Stenella, was found to completely lack Class I receptors, furthermore, the class II receptors this species does possess, exist exclusively as pseudogenes, suggesting that these receptors, again, lost their function during the adaptation to the aquatic environment.

The differentiation between “terrestrial” Class II and “aquatic” Class I receptors has however been recently besmirched by the unlikely discovery of abundant Class I ORs within the human genome (Glusman et ai, 2001). These receptors represent approximately 10% of the entire human OR count, are all confined to chromosome 11, and show a considerably lower pseudogene fraction than that observed for Class II (52% and 77%, respectively). Expression data currently exists for 6 Class I ORs, so it seems that they are under selective pressure

to maintain functional motifs. Within the human genome. Class II families are all present in more than one chromosome each, and so the restriction of Class I receptors to chromosome 11 may indicate the regional control of expression of these genes. No such mechanism has been found for the Class II, and so this would represent a strong functional difference between the two classes.

In microsmatic terrestrial mammals (such as primates), a greater proportion of the OR repertoire is non-functional. Rouquier et al., (2000), isolated and cloned OR sequences from a random sample of primate hominoids and prosimians and compared the percentage of pseudogenes between taxa with that of Mus musculus, a macrosmatic species. A dendogram of these sequences and those available in the literature from other species was constructed to determine any evolutionary relationships. The results suggested that from New World monkeys to hominoids, there is an increase in the percentage of OR pseudogenes, from -0% to -70% (Table 2.1.4).

CHAPTER 2 ISOLATION OF SALMON OR

Familv/sDedes No. sequences % ORF % Dseudoqenes Mean %

Hominoids 50 Human 99 30 70 Chimpanzee 21 52 48 Gorilla 18 50 50 Orangutan 23 61 39 Gibbon 22 59 41

Old World monkeys 27

Macaque 20 65 35

Baboon 21 81 19

New World monkeys 2

Marmoset 19 100 0 Squirrel monkey A 15 100 0 Squirrel monkey B 15 93 7 Prosimians 37 Lemur A 19 58 42 Lemur B 16 69 31 Rodents 0 Mouse 33 100 0 Fish 0 Zebrafish 3 100 0

T ab le 2 .1.4 Fraction o f p seu d og en es in the O R gene repertoire o f prim ate species a n d m ouse. Taken from Rouquier e t ai. (2000).

Although hardly an exhaustive study (only 3 out of a possible 100 zebrafish genes were included), there is evidence of the existence a selective advantage for New World monkeys to retain a greater repertoire of ORs than their Old World counterparts. Similarly, an analysis of the OR cluster on human chromosome 17p13 and a selection of non-human primate orthologs (Sharon et al., 1999) has indicated that a rapid decline (-10 million years ago, corresponding to the radiation of hominids)

in the functional OR repertoire occured in mammals. The authors inferred that all OR genes within this cluster were fully functional before the divergence of orangutans from African apes. Furthermore, it appears that the pool of primate pseudogenes is still growing (Rouquier ef a/., 1998).