Gene-copy patterns track photosynthetic differences in daisy relatives

Plants use carbon dioxide to build sugars, but the enzyme Rubisco can also react with oxygen. That reaction leads to photorespiration, which uses energy. C4 photosynthesis helps limit this problem by concentrating carbon dioxide near Rubisco. It involves coordinated changes in metabolism, anatomy and gene regulation, rather than a change to a single enzyme.

A bioRxiv preprint asks whether an ancient whole-genome duplication is related to the evolution of carbon-concentrating mechanisms in the Heliantheae alliance, a group within the daisy family. In a whole-genome duplication (WGD), an organism acquires an extra set of its genetic material. Over time, some duplicated genes are lost, while others persist and may change in function or regulation.

The researchers generated genome assemblies and annotations for 35 species and included three additional published genomes in broader evolutionary analyses. They used patterns of synonymous DNA changes between gene copies and gene-family histories mapped onto a species tree to identify a shared ancient duplication, which they estimate occurred about 30 million years ago.

To compare genome differences with physiology, the team measured the carbon dioxide compensation point in plants grown under controlled conditions. This is the CO₂ level at which photosynthetic carbon uptake balances carbon loss. The study used it as a proxy for carbon-concentrating mechanism efficiency—not as a complete measure of photosynthesis, plant growth or yield. The paper describes association mapping across species with available measurements, but its Methods also state that two species were excluded because the WGD-2 signal was absent. The exact analyzed sample count is therefore not straightforward to reconcile from the text. The phylogeny-aware method accounts for the fact that related species share evolutionary history.

The analysis identified 451 functional categories, called MapMan BINs, whose gene copy-number variation was statistically associated with differences in the compensation point. These categories included genes related to photosynthesis, photorespiration and gene regulation. Of the 451 associated categories, 303 had at least 10% of their genes traced to the ancient duplication, WGD-2. Separately, a statistical enrichment test found that WGD-2-derived genes were over-represented in these categories compared with the genome-wide background. The 303-category count and the enrichment test are related but distinct findings; neither means every associated gene arose in the duplication.

The relationships did not follow a simple pattern in which each added gene copy corresponded to a steady physiological change. For many categories, the authors’ measure of overall explanatory power was substantially greater than what a linear relationship captured. Transcription-factor categories including HD-ZIP, HSF, LOB and TCP stood out in this analysis. The authors interpret these patterns as consistent with threshold-like effects: copy-number changes may matter in combination or have greater effects at some levels than others. “Tipping point,” however, is an interpretation of comparative statistical patterns, not a switch directly triggered and observed in an experiment.

The study’s evidence is comparative: it relates differences among species to one another. It does not experimentally change gene copy numbers to test whether this causes a plant to acquire C4 photosynthesis. Comparisons across species can also be difficult to interpret because many biological differences may accompany copy-number changes. The authors used a consistent annotation approach, but did not functionally validate individual gene categories. Instead, prediction models tested how well sets of categories predicted the compensation point. Selected associated categories improved prediction performance in some analyses; that supports their usefulness for prediction, but does not establish the function of particular genes.

The findings therefore suggest a possible relationship between an ancient genome duplication, later gene-copy patterns and variation in a physiological proxy for carbon concentration. They do not show that increasing gene copies would improve photosynthesis in sunflower or other crops. The manuscript is a bioRxiv preprint and had not been certified by peer review in the version described. Its proposed evolutionary interpretation should be treated as provisional; experiments on specific genes and regulatory networks would be needed to test causation.