Curtis Lubbe, Institute of Botany of the Czech Academy of Sciences, discusses their article: Trade-offs and bet-hedging in clonal offspring allocation

What about clones?

Most angiosperms reproduce clonally in addition to the production of seeds. That is, they form new units of their bodies (rooting units, or ramets): vegetative clonal reproduction, using rhizomes (horizontal belowground stems) or other structures. Clonal offspring are also a large investment by the plant because they can compose a large part of the plant body and are larger than seeds. While there are many different studies of parental allocation into offspring as seeds (at many different levels and using a variety of metrics), very few studies assess parental allocation to clonal offspring – especially for many species at once (most have one or a few species). Clonal offspring often remain connected and integrated into the body of the parent plant (such as with rhizomes), making it difficult to determine where allocation from a parent begins and ends. There are also many different types of clonal growth organs (such as rhizomes, roots with adventitious buds, bulbs, stem tubers, turions, etc.), with incredible variety among them, so it is difficult to find a definition or method of measurement that fits well for a variety of species at once. Thus, clonal offspring allocation trends remain relatively unknown, even though clonal reproduction is of great importance as a common method of reproduction for many weedy and invasive species, important crops (such as potatoes), lawn grasses, ornamental plants, and more.

The system

The species of southern African Oxalis are an excellent model system for understanding clonal offspring allocation because this is a large group of species (around 230) that all produce tunicate bulbs with no connection to one another or any additional structures once they mature (during summer seasonal dormancy). Therefore, offspring bulbs can relatively easily be counted and weighed, as can support structures used to move the bulbs within the soil (such as belowground stems and contractile roots).

Illustration of 90 of the southern African Oxalis species used in this study, based on observations during bulb collection and measurement. Illustration in ink and watercolour, by F. C. Lubbe. This illustration is also the cover art for the August issue of the Journal of Ecology.

The study

We planted bulbs of over 100 species of southern African Oxalis (representing the taxonomic and trait diversity of the group) into individual pots at a uniform depth. After one growing season (winter), we looked into those pots, weighing bulbs and support structures, and counting bulbs. 

The trends behind clonal offspring allocation 

We found multiple trends behind how these plants allocate to their offspring:

Size/number trade-off: Species that made larger bulbs made fewer of them, and vice versa. This is similar to the size/number trade-off in seeds. Although the size/number trade-off in seeds is very well known, it is quite variable and often not found, depending on the system or the species. Thus, the strength and clarity of the trend found here for clonal offspring is quite noteworthy. This trend has been found for clonal plants intraspecifically, but never interspecifically for a set of many species.

The relationship between bulb size and number among the Oxalis species. The solid line represents the 1:1 regression line; point size indicates parental bulb weight.

Dispersal/protection trade-off: Species with greater investment into overall offspring bulb mass also invested more in spacer stems and roots to spread them through the soil. In contrast, species investing less into offspring bulb mass invested in contractile roots to protect those bulbs using adjustment of depth in the soil. 

Diagram of the bulbs and different support structures produced by southern African Oxalis species (a) and the relationships between investment among them (as percentage of biomass for each structure; b). Illustration by F. C. Lubbe.

Clonal offspring heteromorphism: For many species (around 2/3), individual parent bulbs produced multiple types of offspring, as determined by the production of offspring with different types of traits, here identified by the classification of offspring bulb location (relative to the mother parent bulb). This is similar to seed heteromorphism. Additionally, for a given individual, deeper offspring bulbs (lower placement location, below parental location) were generally fewer and larger whereas shallower bulbs (disperser and upper bulbs, above parental location) were smaller and more in number. 

Offspring placement strategy further grouped into clusters where species within these clusters had more similarity among themselves than species in the other clusters. We found six clusters, two specialized in specific locations (either producing only bulbs in the original parent location, cluster 5, or producing many bulbs spread horizontally from the parental bulb in addition to the parental location, cluster 6). Most species then varied in their placement of bulbs along the vertical axis, either placing bulbs above the parental location, (clusters 3 and 4) or above and below (clusters 1 and 2).

The different clusters of clonal offspring allocation strategy among Oxalis species, based on placement location, relative to the location of the parental bulb. Locations are: disperser (at or above the soil surface), upper (above parental location), consolidator (in original parental location, within old parental tunics), lower (below parental location), and spreader (distanced horizontally from original parental location). Illustrations by F. C. Lubbe.

Takeaways

We found multiple trends in clonal offspring allocation, and importantly, they are all connected. The distribution/protection trade-off connects with the size/number trade-off as the protection of fewer bulbs versus spreading more bulbs. Both trade-offs then coexist within clonal offspring heteromorphism where plants produce more small bulbs at or closer to the soil surface, where they may more easily be spread, or fewer larger bulbs deeper in the soil, where they are better protected. 

Clonality is an important, complex, and dynamic part of plant life – as illustrated here by the multiple, interconnected trends found among the southern African Oxalis species. Clonal offspring are not just formed in addition to seeds and sexual reproduction, they have their own patterns of allocation that expand our understanding of plant reproduction.

For more background about this study, see the blog post on Plant Life Belowground blog, here, or check the Oxalis tag, here, for more discussion, photos, and illustrations of Oxalis.