Summary
A bioRxiv preprint reports genome-pattern evidence for an ancient whole-genome duplication shared by bryozoans. Few duplicate genes remain, but retained copies are enriched for cilia-related genes.
A study posted as a bioRxiv preprint reports evidence that bryozoans experienced an ancient whole-genome duplication. The researchers identified duplicated patterns in the chromosomes of the freshwater species Cristatella mucedo and genome-comparison signatures in eight genomes from the class Gymnolaemata. Their gene-tree analyses support a duplication shared by extant bryozoans, followed by extensive gene loss and separate rediploidisation in two major lineages.
Genome patterns point to an ancient duplication
Whole-genome duplication (WGD) is an event in which an organism acquires an additional copy of its genome. Over evolutionary time, many duplicate genes can be lost, and the chromosome sets can become organised again in a diploid-like state—a process called rediploidisation. That long history of gene loss can make very old duplications difficult to detect.
To search for such events, the researchers compared patterns of gene order, known as synteny. They used both duplicated regions within genomes and comparisons between bryozoan genomes, incorporating bilaterian ancestral linkage groups—reconstructed groups of genes thought to have been linked in an early bilaterian ancestor.
In C. mucedo, the analysis found duplicated paralogous regions across every chromosome. The researchers also report a duplicated Hox cluster, a group of genes involved in regulating animal development. In all eight Gymnolaemata genomes examined, orthologous synteny comparisons showed 2:1 patterns: regions in one genome corresponded to two related regions in another. The authors interpret this, together with their gene-tree results, as evidence that the duplication occurred before the evolutionary separation of the bryozoan groups represented in the study.
Few duplicate genes remain
The genomic signatures persist despite extensive loss of duplicate copies. The preprint estimates that about 10% of duplicated genes remain in C. mucedo and reports roughly 5% duplicate retention in Gymnolaemata. The authors’ gene-tree analyses further support a history in which much of the duplicated genome returned to a diploid-like organisation independently in the two lineages.
Among the retained duplicates, genes associated with cilia are enriched. These genes are preferentially expressed in the lophophore, the bryozoan feeding organ formed by a crown of tentacles covered with specialised cilia. The pattern links some of the surviving duplicate genes to a distinctive structure and its cellular machinery.
A possible link to bryozoan feeding biology
The authors propose that the duplication may have helped bryozoans adapt to a sessile, filter-feeding lifestyle. The cilia-gene enrichment and lophophore expression provide a biological basis for that proposal, but the role of WGD in the evolution of this lifestyle is an interpretation of the genomic and expression patterns, rather than a directly measured effect.
The result also illustrates how comparisons with ancestral linkage groups can help detect ancient duplications after most duplicate genes have disappeared. Because the work is a preprint, its findings are research results that have not yet been presented as peer-reviewed conclusions.