Camouflage as a driver of colour variation and diversity in the ‘stone’ plant genus, Lithops

Scheepers, Paul (2021-12)

Thesis (MSc)--Stellenbosch University, 2021.

Thesis

ENGLISH ABSTRACT: Lithops is a genus of succulent plants that exhibits exceptional diversity in the colouration of its leaves. The menagerie of leaf colours and patterns of the various species often bear a remarkable resemblance to the stones which they grow amongst and this has long been assumed to represent a defensive strategy that allows the plants to avoid the attention of herbivores. Similar rocks or ‘lithologies’ are distributed patchily across the range of Lithops in a mosaic-like pattern due to spatially discrete geological processes. Because camouflage (ie the use of information to fool predator perception and avoid attack) is background-dependent, certain appearances will be optimal for camouflage in specific regions but will be sub-optimal elsewhere. When natural selection favours different genotypes spatially this can lead to a pattern of ‘local adaptation’, which is a pre-requisite for the ecological speciation model. It was the aim of this thesis to assess whether Lithops are indeed camouflaged, as this has never been quantitatively assessed and is believed to be rare among plants. In addition, I aimed to elucidate whether Lithops are locally adapted, which would point towards camouflage as a driver of diversity in the genus. Using digital photography techniques and a model of human vision, I found strong evidence that Lithops are camouflaged and that they tend to match the rocks in their environment better than the soils, although this depended somewhat on which component of colour was analysed (ie luminance versus chroma). I also found that the luminance perceptual distances between Lithops and their backgrounds are much larger than the chromatic perceptual distances. Camouflage might also have played a role in Lithops diversification, as I found evidence of greater camouflage optimization of local populations relative to intra and interspecific foreign populations on different lithologies. Interestingly, optimization of local population camouflage relative to interspecific foreign populations on the same lithology was found but not relative to intraspecific foreign populations on the same lithology. This suggests gene flow counteracts local adaptation of populations within species, or that camouflage relevant variation within lithologies shows a spatial pattern where it is more pronounced across species ranges than within. This thesis provides the first quantitative assessment of Lithops camouflage and its role in the generation of Lithops diversity, and adds to a small but growing body of evidence that camouflage may also be an important defensive strategy in plants.

AFRIKAANSE OPSOMMING: Lithops is 'n genus van vetplante wat uitsonderlike diversiteit in die kleur van sy blare vertoon. Die menasie van blaarkleure en patrone van die verskillende spesies het dikwels 'n merkwaardige ooreenkoms met die klippe waartussen hulle groei en dit is lankal aanvaar dat dit 'n verdedigingstrategie verteenwoordig wat die plante toelaat om die aandag van herbivore te vermy. Soortgelyke gesteentes of 'litologieë' is vlekkeloos oor die reeks Lithops versprei in 'n mosaïekagtige patroon as gevolg van ruimtelik diskrete geologiese prosesse. Omdat kamoeflering (dws die gebruik van seine om roofdierpersepsie te mislei en aanvalle te vermy) agtergrondafhanklik is, sal sekere voorkoms optimaal wees vir kamoeflering in spesifieke streke, maar sal elders sub-optimaal wees. Wanneer natuurlike seleksie verskillende genotipes ruimtelik bevoordeel, kan dit lei tot 'n patroon van 'plaaslike aanpassing', wat 'n voorvereiste vir die ekologiese spesiasiemodel is. Dit was die doel van hierdie tesis om te bepaal of Lithops wel gekamoefleer is, aangesien dit nog nooit kwantitatief beoordeel is nie en glo skaars onder plante is. Daarbenewens het ek daarop gemik om uit te vind of Lithops plaaslik aangepas is, wat sou dui op kamoeflering as 'n drywer van die diversiteit in die genus. Deur gebruik te maak van digitale fotografie tegnieke en 'n model van menslike visie, het ek sterk bewyse gevind dat Lithops gekamoefleer is en dat hulle geneig is om die rotse in hul omgewing beter te pas as die grond, hoewel dit ietwat afhang van watter komponent van kleur ontleed is (dws helderheid teenoor chroma). Ek het ook gevind dat die luminansie perseptuele afstande tussen Lithops en hul agtergronde baie groter is as die chromatiese perseptuele afstande. Kamoeflering kon ook 'n rol in Lithops-diversifikasie gespeel het, aangesien ek bewyse gevind het van groter kamoefleeroptimalisering van plaaslike bevolkings relatief tot intra- en interspesifieke vreemde bevolkings op verskillende litologieë. Interessant genoeg is optimering van plaaslike bevolkingskamoeflering relatief tot interspesifieke vreemde populasies op dieselfde litologie gevind, maar nie relatief tot intraspesifieke vreemde populasies op dieselfde litologie nie. Dit dui daarop dat geenvloei plaaslike aanpassing van populasies binne spesies teëwerk, of dat kamoefleerrelevante variasie binne litologieë 'n ruimtelike patroon toon waar dit meer uitgesproke is oor spesiereekse as binne. Hierdie tesis verskaf die eerste kwantitatiewe assessering van Lithops-kamoeflering en die rol daarvan in die generering van Lithops-diversiteit, en dra by tot 'n klein maar groeiende hoeveelheid bewyse dat kamoeflering ook 'n belangrike verdedigingstrategie in plante kan wees.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/123842
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