====== CYP20 clan ======


P450 sequences of the CYP20 clan are widely but not uniformly distributed in animals. The function of CYP20 remains enigmatic and even the function of the human CYP20A1 gene is still unknown. 

A FASTA file of currently identified CYP20 clan sequences and partial sequences from arthropods is found {{ ::cyp20sequences.fasta |here}}. 

The presence of a CYP20 homolog in the damselfly, //Calopteryx splendens// and in copepods was reported by Ioannidis et al. (2017) and Han et al. (2017). CYP20 sequences are found in genomes and transcriptomes of various, but not all taxa of Chelicerates, Myriapods and Pancrustacea. See Figure 3 of Dermauw et al., 2020.{{ ::cyp20distribution.jpg?400|}}(stippled line: no CYP20 in that lineage)
 
Among Chelicerates, CYP20 sequences are found in ticks, but not in mites, except oribatid mites. CYP20 are found in some Aranea, //Acanthoscurria geniculata// (as intronless gene) and //Latrodectus geometricus//, but not in //Parasteatoda tepidariorum//. They are also found in all scorpions and horseshoe crabs (Xiphosura). 

CYP20 sequences are found in myriapods, both Chilopoda (//Strigamia maritima// and //Scolopendra dehaani//) and Diplopoda (//Chamberlinius hualienensis//, //Glomeris pustulata//, //Polydesmus angustus//, //Helicorthomorpha holstii// and //Trigoniulus corallinus//).
 
In “Crustacea”, CYP20 sequences have a very patchy distribution, perhaps owing to the limited genomic coverage (when compared to insects). They are found in copepods, //Tigriopus japonicus//, //Paracylopina nana// and //Eurytemora affinis//, in //Gammarus fossarum// (amphipod) and in barnacles (Cirripedia) but not in //Hyalella azteca// or //Daphnia pulex//. They are also found in Branchiura and Ostracoda. 

No CYP20 is found in Collembola, but CYP20 sequences are found in Diplura and Protura. In insects, CYP20 are restricted to Archaeognatha, Zygentoma, Odonata and Ephemeroptera. 

In most cases, CYP20 is a single gene, but there are six genes in the centipede //Strigamia maritima// and two in the millipede //Polydesmus angustus//. The CYP20 clan P450s appear to be polyphyletic in arthropods because the gene tree does not follow the species tree. The patchy distribution of CYP20 homologs indicates multiple losses in arthropods. 




The CYP20 sequences differ from usual P450 sequences. The typical FxxGxxxC Cys pocket motif has a codon insertion between Phe and Gly (three codons instead of two). In oribatid mites there is a further codon insertion between Gly and Cys. Pro always follows the Cys codon and while this is not unique to CYP20 sequences, it is relatively uncommon. In Cirripedia, the CYP20s have lost the conserved Cys altogether.{{ :cyp20.jpg?400 |}}