PEPPER, ALAN E*, HEATHER HERRICK, and LAURA E NORWOOD. Department of Biology, Texas A&M University, TAMUS 3258, College Station, TX 77843, USA. - Evolution and population structure of Caulanthus amplexicaulis var. barbarae (Brassicaceae), a rare serpentine endemic plant.
Serpentine soils are characterized by low levels of the essential
plant nutrients nitrogen, phosphorus, potassium, and calcium, as well
as high levels of iron, magnesium, and manganese, and toxic levels of
chromium, cobalt, and nickel. Serpentine-derived soils have been found
to be a significant impetus for plant speciation and endemism
worldwide. The Santa Barbara Jewelflower, Caulanthus
amplexicaulis var. barbarae (J. Howell) Munz
(Brassicaceae), is a rare herbaceous annual plant restricted to an
archipelago of serpentine exposures in the San Rafael mountains of
central California, USA. Its sister taxon, C. amplexicaulis
var. amplexicaulis S. Watson, has a more widespread
distribution in southern California. C. amplexicaulis var.
amplexicaulis is largely restricted to granitic soils. Intra-
and interspecific phylogenetic relationships Caulanthus
amplexicaulus var. barbarae and related taxa in the
"Streptanthoid Complex" of genera (Streptanthus,
Caulanthus, Guillenia) were examined using nuclear ribosomal
internal transcribed spacer (ITS) and chloroplast trnL intron
sequences. Phylogenetic hypotheses supported monophyly of subgenus
Euclesia — which includes the bulk of the serpentine endemics
in the Streptanthoid Complex — but showed that as a group, the
serpentine tolerant taxa were nonmonophyletic, occurring in at least
three distinct clades. Extant population structure and recent
population dynamics in C. amplexicaulus var. barbarae
were examined using a combination of microsatellite loci and nuclear
ITS sequence variation. These studies suggest a role for fire in the
long-term maintenance gene flow among isolated populations on
serpentine outcrops. Although ecologically and geographically
isolated, C. amplexicaulis var. barbarae and C.
amplexicaulis var. amplexicaulis remain fully interfertile
in artificial crosses. The existence of genetically compatible taxa
with such distinct edaphic requirements presents a unique opportunity
for intensive study of the genetic basis of tolerance to serpentine
soils.
Key words: Caulanthus amplexicaulis, chloroplast trnL, edaphic, microsatellite, nuclear ITS, speciation