Peter Breslin

Peter Breslin

Dec 28, 2017

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Hooks

One of the traits shared by most of the Baja California Mammillaria (Cactaceae), the genus I am studying, is strongly hooked spines. It is almost certainly the case that hooked spines across the Cactaceae is not a synapomorphy, that is, a trait shared by two or more descendants of a common ancestor. Rather, it seems that hooked spines evolved many times over, across genera, that is, that it is a homoplasy, or convergence. But it is unknown whether or not subgroups within Mammillaria, such as the Baja California Mammillaria, share hooked spines as a synapomorphy.

No one has advanced a very comprehensive or convincing explanation of the advantage of hooked spines in cacti. In some ways, on first reflection, those hooks even seem like a possible detriment. Animals small and large often uproot plants, the hooks buried in their hair or flesh. Many of the hooked spine plants in the Cactaceae, oddly enough, are not set up for vegetative reproduction— that is, most (including the Mammillaria) rarely become established in a new locale as a result of being dragged around, stuck to a rat or other animal. A great many cacti rely on this clonal stem reproduction that is usually accomplished by straight spines (often, as in the chollas, retrorsely barbed for both maximum stickiness and extreme pain). In fact, when you are in the desert and see a huge "forest" of a cholla such as Cylindropuntia bigelovii, what appears to be many individual and distinct plants can often be just a huge field of genetically identical clones.

Not so with Mammillaria, which as a rule do not re-root if dragged out of their soil, at least not in my personal observation over the past 30 years in the field. Suggested advantages of hooked spines include channeling condensed dew to the root zone more efficiently and providing extra protection. The basic physiological mechanism wherein hooked spines are grown by a cactus plant is understood— a differential elongation of cells in the maturing tip of the spine. In general, hooked spines first emerge as tiny straight spines. The hook forms quickly in most species as the spine grows out of the areole.

The topic of hooked spines versus straight spines sometimes takes on importance when plant systematists are working on developing phylogenies or taxonomies. Even minor variations in the "strength" of the hook have been used to try to distinguish taxa. In the group of plants I am studying, sometimes referred to as Cochemiea, all of the forms have strongly hooked spines, sometimes more than one per areole, except for the main study species with which I am working, Cochemiea halei. It's interesting to me that a fairly well defined subgroup of peninsular and island endemics has developed this contrast. Perhaps in sorting out the molecular phylogeny and biogeography of the Baja Mammillaria, some light will be shed on the importance of hooked spines both in an ecological and a taxonomic sense.

Four forms of Cochemiea that have been recognized in the past, all with hooked spines, clockwise from upper left: Cochemiea pondii, Cochemiea poselgeri, Cochemiea setispina and Cochemiea maritima

Cochemiea halei, with straight spines.


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About This Project

I am developing and applying innovative population genetics data analysis with habitat suitability modeling to solve longstanding challenges in the effort to save endangered plant species. Combining high throughput RADseq data analysis with species distribution models, I am exploring relatively inexpensive, feasible methods to generate powerful population viability assessments, estimates of threshold population size and the constraints on the habitats of rare plants.

Blast off!

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