It's the hard-knock life

... if you're a small plant living in a desert – it's hot and dry, and the surroundings teem with plant-feeding mammals, birds, and insects. Because of long dry spells, you have to store water in your tissues, which makes you all the more interesting for the local animals. In the New World, cacti defend themselves against being eaten by growing sharp spines, but spine production is costly.

Plants in the southern African genus Lithops instead rely on camouflage: each plant grows only two thick leaves that are for the most part buried inside the soil. Only the drab-colored flat tops of their succulent leaves protrude to the soil surface, making Lithops plants virtually invisible for visually searching enemies. This astonishing resemblance to small pebbles has earned them the name "stone plants".

It's not over yet: enter geographic variation in soil color

Fine. You live a happy, semi-subterranean life somewhere in southern Africa, flower each year and produce copious amounts of offspring that are equally cryptic. The problem is, however, that the properties of the soil vary markedly across the arid regions of South Africa, Namibia, and Botswana: the gravel can be brown, grey, yellow, red, or almost white, and the grain size likewise ranges from fine sand to large pebbles. After many generations, but sooner or later, some of your descendants will therefore find themselves in a hostile environment where they stand out from their surroundings. Back to square one?

Here we come to the core of the evolutionary theory developed by Charles Darwin and Alfred Russel Wallace over 150 years ago: if upper-surface color varies among Lithops individuals, if the coloration is genetically determined and heritable, and if poorly camouflaged individuals get eaten more often than do cryptic ones, then the color and patterning of the plants will over time tend to converge towards the visual properties of the local soil.

From local adaptation to diversification?

One of the most central questions in modern evolutionary research concerns the role of ecological traits in the origin of new species. Speciation follows from the build-up of genetic incompatibilities between separate populations or lineages, which in most cases requires a long period of low or no interbreeding across the nascent species boundary. Interestingly, adaptation of geographically separate populations to contrasting environmental conditions or resources is one possible factor that could limit gene flow across populations, and therefore could enhance the likelihood of speciation.

There are approximately 37 described species in the genus Lithops, and many of them are very variable when it comes to overall coloration and the reticulated patterning of their upper surfaces. The hypothesis that diversification of stone plants has been driven by local adaptation to geographically variable soil color is a very intriguing – but hitherto untested – hypothesis.

Lithops: the ultimate model system for evolutionary research?

In the Lithops project, we aim to use stone plants as a model system in research on the evolution of locally optimized crypsis and the formation of new species in the arid regions of southern Africa, which constitute one of the Earth's main hotspots of plant biodiversity.

We will start our project with two field expeditions to South Africa and Namibia in April 2016 and 2018. During the expeditions, we will use hyper- and multispectral camera systems to objectively quantify the extent to which the color and patterning of Lithops species matches the visual appearance of the soils of their distributional range. These data will be used to test the hypothesis that phenotypic (color) divergence within the genus Lithops is driven by selection for locally optimized camouflage. We will also conduct background-altering experiments in order to investigate whether individuals that stand out from their surroundings face an increased risk of being eaten. During the field expeditions, we will take genetic samples of the imaged plants, so that the underlying genetic basis of color production can be studied in the future.

Progress in evolutionary research is in many cases based on model systems that present an opportunity to test ecological and genetic hypotheses in a relatively simplified setting. Important model systems typically consist of groups of closely related species, such as threespine sticklebacks, Trinidadian guppies, and monkeyflowers (Mimulus). Another feature of most model systems and organisms is that they are relatively small and easy to breed or cultivate in a lab or a greenhouse. Sounds an awful lot like Lithops, doesn't it?

Links

Lithops can arguably be described as the weirdest plants on the planet, but their peculiarity and strange charisma have made them popular among enthusiasts around the globe. Below are a few excellent websites on Lithops taxonomy, biology, and cultivation. The list is far from comprehensive, so if you notice that something is missing, please send us a note and we'll correct the omission!

 

Flowering stones: www.floweringstones.co.za

Lithops info: www.lithops.info/en

Interactive distribution map: http://mesemb.ru/map

Wikipedia: https://en.wikipedia.org/wiki/Lithops

Project funding

Our Lithops imaging expeditions are supported by a grant from National Geographic Society Science and Exploration Europe.

 

 

 

Hyperspectral camera equipment for the second expedition in 2018 was provided by Specim Spectral Imaging Ltd. (Oulu, Finland).

 

 

 

Hyperspectral camera equipment of the first expedition in 2016 was provided by the Surface Optics Corporation (San Diego, CA).