Insights into the evolution of the eucalypt CER1 and CER3 genes involved in the synthesis of alkane waxes

The genes ECERIFERUM1 ( CER1 ) and ECERIFERUM3 ( CER3 ) encode the biosynthesis of alkane waxes, a key component of the plant cuticle. To study the evolution of CER1 and CER3 in a highly diverse group of eucalypts, we performed a genome-wide survey using recently released genome assemblies of 28 Myrtaceae species, with 22 species from the main eucalypt lineage and 6 non-eucalypt Myrtaceae tree species. We manually annotated 250 genes and pseudogenes, identifying a near-ubiquitous single copy of CER3 and 2 to 10 CER1 gene copies per Myrtaceae species. Phylogenetic analysis suggested that copy number variation in eucalypts is due to multiple tandem duplication events, both ancient (shared by all Myrtaceae species studied) and relatively recent (present only in eucalypts). Inter-chromosomal translocations were discovered for both CER1 and CER3 , along with recurrent loss of often the same CER1 introns in the WAX2 domain, the domain that is essential for wax production. Despite the varied environments occupied by the eucalypt species in this study, we did not find statistically significant associations between intra-genic structural changes or CER1 copy number and aspects of the environment they occupy (including aridity). The challenge is now to explain the species-specific evolutionary histories that contributed to the observed variation in CER1 and the extent to which it may contribute to the adaptability of eucalypts.