Taxonomy & Evolution

Piecing together the Zamiaceae family tree reveals the evolution of how plants ‘breathe’

The Zamiaceae are often considered 'living fossils', but a close study of how they have evolved over millions of years can give a glimpse into the lives of past plants.

If you could examine the common ancestor of seed plants, then you could get insights into why some survived and some didn’t. A problem is that this common ancestor no longer exists, so how could you find out what it was like? One method is to look at surviving ancestors and see what traits they share.

Mario Coiro and colleagues have been examining the cycad family Zamiaceae. Cycads diverged from other plants when seed-plants were just starting, so looking at them and finding what traits they share with other plants can yield clues as to what this common ancestor was like.

Photographs of germinated seedlings of (A) Bowenia spectabilis, (B) Dioon edule and (C) Macrozamia communis. Source: Coiro et al. 2021.

The Zamiaceae are often considered ‘living fossils’, which is a bit of a misleading name. A fossil is preserved in stone, so it shows a plant as it was. The Zamiaceae have been around many millions of years, and they have developed over that time. So Coiro and colleagues examined them to see how the various family members are related to each other. If you know the family tree, you can deduce what traits are plesiomorphic, shared with the common ancestor, and what are innovations.

The team specifically looked at stomata, the holes in the leaves that allow air to enter and leave. The cells around the stomata can be helpful in working out how plants developed. Crucially, stomata can also be preserved in fossils, allowing you to look back at extinct species. A problem is that cycad stomata are three-dimensional. Often the stomata are sunk in ‘stomatal pits’, which makes them difficult to image. Coiro and colleagues tackled this by using light microscopy and confocal microscopy to get a close look.

One of the features the team found that was plesiomorphic were perigenous encircling subsidiary cells. Perigenous in this case, means that the cells were derived from a single mother cell. However, the way the lateral cells are divided into subsidiary and encircling cells suggests that stomatal development is a bit more complicated than the usual classification of ‘haplocheilic’ and ‘syndetocheilic’ stomata.

RESEARCH ARTICLE

Coiro, M., Barone Lumaga, M.R., Rudall, P.J., 2021. Stomatal development in the cycad family Zamiaceae. Annals of Botany. https://doi.org/10.1093/aob/mcab095

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