Avian surfactant protein (SP)-A2 first arose in an early tetrapod before the divergence of amphibians and gradually lost the collagen domain

The air-liquid interface of the mammalian lung is lined with pulmonary surfactants, a mixture of specific proteins and lipids that serve a dual purpose-enabling air-breathing and protection against pathogens. In mammals, surfactant proteins A (SP-A) and D (SP -D) are involved in innate defence of the lung. Birds seem to lack the SP-D gene, but possess SP-A2, an additional SP-A-like gene. Here we investigated the evolution of the SP-A and SP-D genes using computational gene prediction, homology, simulation modelling and phylogeny with published avian and other vertebrate genomes. PCR was used to confirm the identity and expression of SP-A analogues in various tissue homogenates of zebra finch and turkey. In silico analysis confirmed the absence of SP-D-like genes in all 47 published avian genomes. Zebra finch and turkey SP-A1 and SP-A2 sequences, confirmed by PCR of lung homogenates, were compared with sequenced and in silico predicted vertebrate homologs to construct a phylogenetic tree. The collagen domain of avian SP-A1, especially that of zebra finch, was dramatically shorter than that of mammalian SP-A. Amphibian and reptilian genomes also contain avian-like SP-A2 protein sequences with a collagen domain. NCBI Gnomon-predicted avian and alligator SP-A2 proteins all lacked the collagen domain completely. Both avian SP-A1 and SP-A2 sequences form separate clades, which are most closely related to their closest relatives, the alligators. The C-terminal carbohydrate recognition domain (CRD) of zebra finch SPA1 was structurally almost identical to that of rat SP-A. In fact, the CRD of SP-A is highly conserved among all the vertebrates. Birds retained a truncated version of mammalian type SP-A1 as well as a non-collagenous C-type lectin, designated SP-A2, while losing the large collagenous SP-D lectin, reflecting their evolutionary trajectory towards a unidirectional respiratory system. In the context of zoonotic infections, how these evolutionary changes affect avian pulmonary surface protection is not clear.

Automated summary

The air-liquid interface of the mammalian lung is covered by pulmonary surfactants, while birds possess different genes related to surfactants. This study investigated the evolution of these genes and confirmed the presence of SP-A analogues in zebra finch and turkey. The absence of SP-D-like genes in avian genomes was confirmed, and the SP-A1 and SP-A2 sequences in birds were found to form separate clades closely related to alligators. Birds retain a truncated version of mammalian type SP-A1 and a non-collagenous C-type lectin, while losing the large collagenous SP-D lectin.