Abstract

Previous descriptions of natural hybrids between the Ariocarpus species are few, and those that exist are sparse in detail or photographic evidence, or are based on a single specimen. This is the first time that a significant population of natural hybrids has been studied. Below are described a range of hybrids between A. agavoides and A. kotschoubeyanus in the environs of Tula, Tamaulipas. Hybridisation in this population is ongoing with specimens of all ages and morphologies being observed.

Previous Records of Natural Hybrids in the genus Ariocarpus

Hybrids of Ariocarpus are easily created in cultivation, all species being compatible to varying degrees. In habitat, however, hybrids are few and far between. The only reports of natural hybrids of Ariocarpus known to the authors are the badly documented A. x drabii, described by J. Halda and J. Sladkovský from near Dr Arroyo, Nuevo Leon, a hybrid of A. kotschoubeyanus x A. retusus, and a cross between A. kotschoubeyanus and A. fissuratus lloydii from Viesca, Coahula,  illustrated by P. Pavlíček in the Kaktusy Ariocarpus Special ¹. Anderson and Fitz Maurice ^{2, 3} believed that the form of A. retusus ‘confusus’ found near Aramberri, Nuevo Leon, constituted a hybrid swarm between A. retusus and A. trigonus. After extensive observation of these plants in the field our thoughts are that they are one of the many variants of the A. retusus complex rather than hybrids.

Hybridisation in the Tula Region

In October 2003 the authors discovered a small area near Tula, Tamaulipas, where a significant number of both Ariocarpus agavoides and A. kotschoubeyanus ‘albiflorus’ plants were growing sympatrically, often within a few centimetres of each other, Figure 1a and Figure 1b. A. agavoides had clearly migrated from its typical hillside habitat into this part of the flat gypsum plain A. kotschoubeyanus habitat. This finding prompted an intensive search for hybrids, but none were found. However, later close examination of photographs taken at the time revealed a hybrid plant, not recognized as such during the visit, which resembled A. agavoides but with a central grove along the tubercles, Figure 2.

This discovery prompted a search for hybrids on subsequent visits. A visit in February 2004 combed the area but was unsuccessful in locating any hybrid plants, due to the plants having retracted into the ground during the dry season and being covered by the previous year’s fall of dead mesquite leaves. However, a further visit in October of that year, when the plants were in growth, was much more successful and an area was located with plants of species and hybrids, Figure 3 and Figure 4. Amongst the hybrids, plants that more resembled A. kotschoubeyanus, having short tubercles but longer than those the species, occurred, Figure 5a, Figure 5b, and Figure 5c and also others that more resembled A. agavoides, but with shorter tubercles than the species and an areolar grove, Figure 2, Figure 6a and Figure 6b. The hybrid plants were very limited in numbers and over a period of several hours about twenty five were observed.

Ariocarpus hybrids produced in cultivation tend to have more of the morphological features of the male parent. If this applies to these wild plants, it would suggest crossing in both directions has occurred, namely. A. kotschoubeyanus x A. agavoides and A. agavoides x A. kotschoubeyanus. All the observed flowers of hybrid plants were pink in colour, again this is what would be expected based on artificial hybrids in cultivation where pink is usually dominant in crosses between species with differing flower colours.

The habitats of both species are threatened by human activity associated with their proximity to the expanding town of Tula, that of A. kotschoubeyanus, in particular is in demand for housing and cultivation, whilst the A. agavoides habitats have been reduced by quarrying and by erosion caused by over grazing.  The area containing the hybrids is currently grazed by goats, but could be threatened by building activity in the future.

Possible Future Discoveries

There is little doubt that more hybrid populations of Ariocarpus species await discovery. It is suggested that a likely cross is A. agavoides x A. retusus - these two species occur sympatrically in parts of the A. agavoides range ⁴. In the state of San Luis Potosí A. bravoanus ssp. hintonii occurs sympatrically with A. retusus near Matehuala, although searching has so far yielded no hybrid specimens. There are also many locations that so far have not been subject to systematic survey and probably others that are as yet undiscovered.

References

1. L. Kunte & V. Šedivý, Kaktusy XXXVII Special 2, 2002, pp. 26-7.
2. E. F. Anderson & W.A. Fitz Maurice, Haseltonia, 5, 1997, pp. 14-15.
3. E. F. Anderson, Cact. & Succ. J. (U.S.), 71, (4), 1999, pp.189-90.
4. J. M. Sotomayor, A. Arredondo and M. Martinez, Kakteen und Andere Sukkulenten, 51, (5), 2000, 113-118.

Acknowledgements

The authors wish to thank Martin Smith, Nancy Linge, David Rushforth, and Ken Scales for their help in searching for these plants.