Santa Rosa Plateau Hybrid Brodiaea)
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Hybrid Brodiaea On The Santa Rosa Plateau
© W.P. Armstrong 12 May 2009

Themidaceae: Brodiaea terrestris ssp. kernensis x B. santarosae or B. filifolia

This small clonal population on Mesa de Colorado appears to be sterile hybrids between B. terrestris ssp. kernensis (BTK) and Brodiaea santarosae which grow intermixed in certain areas of the Santa Rosa Plateau. The large, strap-shaped staminodes resemble BTK; however, the slender, apical extensions resemble Brodiaea santarosae. Another possible parent is B. filifolia; however, the staminodes of the later species are threadlike (filiform). The anther sacs of these putative hybrids generally are devoid of pollen, with only cellular debris and some malformed pollen grains. The hybrids undoubtedly represent a clonal population that has reproduced asexually from cormlets. The best hypothesis to explain the pollen sterility is non-homologous parental chromosomes resulting in synaptic failure during microsporogenesis. The most likely parental species (BTK and B. santarosae or B. filifolia) have different chromosome numbers. The dissimilar maternal and paternal chromosome sets in the hybrid fail to pair up properly during synapsis of meosis I. Numerous flowers in this population were carefully examined during a period of three years, and they were all sterile. The presence of viable pollen in one flower sample from this location is unexplained at this time.

The above anthers are undersized and devoid of mature pollen grains. This is to be expected in hybrids between parents with different chromosome numbers, such as BTK and B. santarosae or B. filifolia. Tentative chromosome counts by this author for coastal BTK in San Marcos are at least 36. Tentative chromosome counts by Dale McNeal of University of the Pacific for coastal BTK on Otay Mesa appear to be greater than 40. Niehaus (1971) gives chromosome counts of 48 for BTK in Kern County. His number of 36 for B. jolonensis on Otay Mesa is undoubtedly from coastal BTK. The diploid number for B. santarosae is not known precisely at this time, but tentative counts by Annette Winner of the San Diego Natural History Museum indicate that it is in the low 20s. My observations of microspore mother cells of B. santarosae indicate a chromosome number much lower than BTK. According to Niehaus (1971), the diploid chromosome numbers of B. filifolia and B. orcuttii are 24. Based on similar floral morphology, the chromosome number for B. santarosae is probably closer to the latter two species. Visible pollen grains on the perianth segments in the above images undoubtedly came from insects, including small beetles and flower bees, or possibly by the wind.

Pollen mother cell (microsporocyte) of Brodiaea santarosae during prophase I of meiosis. The diploid chromosome number is in the low 20s, similar to B. filifolia and B. orcuttii, much lower than B. terrestris ssp. kernensis. Slide stained with acetocarmine. Magnification 1000x.

Potted Brodiaea Hybrid From SRP "Perc Pit" Photographed 12 May 2009

Flowering hybrid grown in San Marcos gabbro. The flower has the typical 3-pronged staminodes like those at the "perc pit" on the Santa Rosa Plateau. This flower meets the criterion of sterile because the anthers lack pollen. Microscopic examination of the anther contents reveals clear liquid, cellular debris and scattered malformed pollen grains. The scape was 23 cm tall, taller than the small clonal population on the Santa Rosa Plateau. Potted brodiaeas with weekly watering often grow taller than wild plants, especially during drought years.

Left: Anther (A) and staminode (B) of hybrid brodiaea. B. Magnified view (400 x) of anther contents showing a few scattered, malformed pollen grains and cellular debris. The needle-like structures are raphide crystals of calcium oxalate. Slide stained with safranin.

Magnified view (100x) of anther contents showing a few scattered, malformed pollen grains and cellular debris. The needle-like structures (black arrow) are raphide crystals of calcium oxalate. Slide stained with lactic acid orcein.

Fertile Pollen of Brodiaea santarosae & BTK

Random view of the pollen of Brodiaea santarosae. In the above four quadrants there are 9 blank grains (marked with red x's) and 60 normal grains, a pollen viability percentage of 85%. I did ten random pollen samples like this one and got percentages as low as 80% and as high as 90%. The pollen was stained with acetocarmine. Magnification 100x.

Coastal BTK (Brodiaea terrestris ssp. kernensis): More than 97% of the pollen grains are fertile. Fewer than 3% are smaller, transparent blanks. Mature grains are approximately 50 micrometers (0.05 mm) long. An unfortunate flower thrip was dining on the anthers when the pollen sample was taken. This animal became part of the stained slide (lower left quadrant). The pollen was stained with acetocarmine. Magnification 400x

A sterile hybrid Brodiaea in San Marcos between Coastal BTK (B. terrestris ssp. kernensis) and B. filifolia or B. orcuttii has very different staminodes. The staminodes are flat (strap-shaped) or slightly hooded at the apex. They do not have an elongated middle tooth like the Santa Rosa Plateau hybrid. Perhaps one of the parents is B. orcuttii rather than B. filifolia? See the following image:

This is a hybrid between Coastal BTK (Brodiaea terrestris ssp. kernensis) and B. filifolia or B. orcuttii which occur nearby. In fact, both B. filifolia and B. orcuttii appeared in a flower pot with corms of the transplanted hybrid. Note the strap-shaped staminodes that are not hooded as in typical coastal BTK. The flower was completely sterile with no mature pollen grains, giving credibility to the hybrid hypothesis.

  A Sterile Brodiaea Hybrid In San Marcos  
Diploidization & Origin Of Sterile Hybrid

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