Jim Shields' Garden Notes
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Blog Home : January 2008

- Winter Doldrums

Bloom is sparse now, the weather is mostly not encouraging, and it's a great temptation to wax philosophical. I'll resist that temptation. Blooms on the orchids we bought before Christmas are fading on the two plants kept at room temperature (the Slc. hybrid and the complex Odontoglossum hybrid) while the one kept in the cooler entry way, at about 55°F, is still fresh looking. Two of the Dendrobium hybrids in the greenhouse have a few blooms open, while the Oncidium-types just have spikes half grown.

The blooms on the Haemanthus pauculifolius are fraying, and they will be the last Haemanthus to bloom until H. montanus blooms in June. At the same time, Scadoxus puniceus are starting to show signs of life, and one of them has a scape half way up. They usually bloom somewhere between late January and early March in the cool greenhouse (low temperatures dip into the high 30s F).

In the warm greenhouse (pretty constant at about 55 ° F) only Nerine undulata is in bloom. We won't see any new Nerine flowers now until sometime in July, once the undulata fade. I've had some small N. humilis growing for several years, but they do not seem to get much larger, they do not flower, and I suspect they have a viral or a fungal disease. I will probably have to get rid of them.

My Nerine sarniensis hybrids are looking good. Taking a tip from Nick de Rothschild, I tried feeding them, using soluble fertilizer with 0% nitrogen. They have perked up tremendously! I formulated my own fertilizer, using finely crystalline reagent grades of Dibasic Potassium Phosphate (i.e., K2HPO4 ), Monobasic Potassium Phosphate (i.e., KH2PO4 ), and Potassium Sulfate (i.e., K2SO4 ), very thoroughly dry mixed. I used equal parts by weight of each of the three chemicals, giving a moderate level of phosphate (P) and a high dose of potassium (K). I dissolve about ½ teaspoonful of the mixture in 1200 mL (about 5 cups) of water. We have to avoid using more than tiny traces of nitrogen on the broadleaf Nerine species and hybrids, because feeding them nitrogen seems to release latent virus infections that they all carry. You can ruin a collection of broadleaf Nerine (i.e., hybrids of Nerine bowdenii, sarniensis, and several others) by dumping a little bit of nitrogen on them.

Nick is selling bulbs of some of his sarniensis hybrid nerines to other breeders and a few serious collectors this year. I'm not sure I'm serious enough to risk buying such rare and valuable bulbs. Nick's nursery is called Exbury Nerines and the web site is http://www.nerines.com if you are curious about what he has to offer.

Outdoors, thanks perhaps to two or three sunny days of April-like temperatures with afternoon highs in the high 50s to low 60s F a week or so ago, I found a few green noses of Trillium nivale and quite a few small green shoots of Galanthus nivalis showing above ground in the woodland garden. Otherwise, the outdoors here looks just as one would expect for January in Indiana.

Good gardening,

Jim

- How to Make a Flower

I've been brushing up on the ABC's of flower formation, and I want to see if I understand it well enough at this point to explain it to anyone else. Readers of this blog are therefore going to be my guinea pigs.

I'm using mostly the names of the genes as they were described for the dwarf cress, Arabidopsis thaliana, which is the "lab rat" or "fruit fly" for plant biologists. Arabidopsis thaliana is a small species, easy to propagate in the lab, with a few large chromosomes, and one from which it has been possible to isolate a large number of mutants.

Almost 20 years ago, it was found that there are a set of specific genes that lead a new shoot to form a flower. Because there were initially three general types found, they were referred to as the A, B, and C classes. If you inactivated certain of the genes, you did not get any petals in the flower. This was therefor called APETALA. When it was found that there were two genes that caused loss of petals when mutated, they ended up with APETALA1 (AP1) and APETALA3 (AP3). It turns out that you need two classes of genes to make petals: A and also B. APETALA1 is the A gene and APETALA3 is one of the B genes. APETALA probably comes from "a-" meaning without and "petal." A second B gene was found, and given the name PISTILLATA (PI). I haven't tracked down where that name came from. It turns out that there have to be functioning copies of both the B genes in order to have full, normal flower development. APETALA3 and PISTILLATA form what is called a "heterodimer," e.g., a structure made up of two protein molecules that are not identical.

The C gene is necessary for formation of the sexual parts of the flower. When it is mutated, there are no stamens and no pistil, so it was named AGAMOUS (AG). If one of the B genes is mutated, you get the pistil but no stamens. The formation of stamens therefor requires functional B genes and the C gene. The C gene alone is needed for formation of the pistil.

When the A, B, and C genes are expressed ectopically (in non-flower parts of the plant), there are no flower parts formed. This indicated that more genes are needed to get sepals, petals, stamens, or pistils than just the ABC set.

ABC Genes (c) 2007 by Shields GArdens Ltd.  All rights reserved.
The Scheme for Floral Gene Interactions

Those additional genes are now referred to as the SEPALLATA (SEP) genes, and are often called the E class of genes. There are 4 of these genes, SEP1, SEP2, SEP3, and SEP4. These function somewhat redundantly, but in the absence of all 4 of the SEP genes, there are no flower parts produced. Which SEP genes are expressed depends on when and where one looks. The discovery that a fourth class of genes is required has led to recent papers calling this the "ABCE" model.

The "quartet model" for how these gene product work is based on studies of protein-protein interactions. The quartet AP1-AP1-SEP-SEP leads to formation of sepals. The quartet AP1-AP3-PI-SEP leads to petals; AP3-PI-AG-SEP induces stamens; and AG-AG-SEP-SEP forms carpels (ovary and pistil). Protein chemists would refer to these quartets as tetramers, since the four separate protein molecules involved are physically associated with each other.

All of these genes are members of the MADS-box family (except for AP2). The MADS structure is named for the four very different species in which such genes were found: Mouse, Arabidopsis, Drosophila (fruit fly), and Saccharomyces (yeast). MADS genes have also been found in mammals, including humans. This is clearly a very old motif in the history of life.

A paper published in 2007 on this is Soltis et al., The ABC Model and its Applicability to Basal Angiosperms, in ANNALS OF BOTANY, vol. 100, pp. 155163. I also found a slightly older paper, Zahn et al., The Evolution of the SEPALLATA Subfamily of MADS-Box Genes..., in GENETICS, vol. 169, pp. 2209-2223 (2005) to be quite useful. If you search in Google Scholar using advanced search mode and including only recent years, you can find a large number of recent papers on the genetics of floral identity and determination. Try using search words like ABC + Floral, or MADS-box, or SEPALLATA, for instance.

Good gardening,

Jim

- Speciation

Speciation is the process in nature whereby new species arise from existing species. This has been a point of contention among evolutionary biologists since Darwin's time. I think the issue is becoming clearer, but plant biologists are still not completely agreed on the mechanisms.

In the early 1980s, I read a fascinating little book by Vern Grant, called "Plant Speciation." It laid out, in what I thought was a fairly straight forward way, where new plant species might come from. Among the cases in point were the rain lilies. Grant used them as examples of new species arising from polyploidy and from parthenogenic mechanisms.

Now there is another book on speciation, which I just received my copy of: "Speciation" by Coyne & Orr, published in 2004, so really quite current. You can find it in Bookfinder, Barnes & Noble on-line, and probably other places. The cost seems to be about $60 (US), new or used, for the soft cover edition. It's called a textbook, but there are no "exercises" at the ends of the chapters.

As I work my way through it, I'm going to bounce my impressions of the material off readers of my blog. Maybe that way I'll absorb more of it. More importantly, I'll be very interested in the comments of others who have an interest in speciation, so please share your ideas through the blog e-mail link.

The conclusions seem to be, at first glance, that Ernst Mayr's definition of the "Biological Species Concept" is holding up well, with some emendations as we learn more detail about the subject. The Biological Species Concept was that a species is a population or populations of interbreeding individuals. It is also concluded that recent evidence points to natural and sexual selection being the dominant forces in speciation, and that genetic drift plays only a minor role. The authors conclude that allopatry, while important in most cases, is not always necessary for speciation. It is even concluded that "species" is a real entity, as opposed to being just an arbitrary rationalization created solely by the human mind for purposes of organizing information. I've sometimes not been so sure about that myself.

I'll be interested to see how the book deals with the dynamic aspects of species and populations. That is the part that fascinates me -- can we ever step twice into the same river of species?

If you are interested in the current state of the process of speciation, I think this is going to be the book that you must read.

"Speciation" by Jerry A. Coyne and H. Allen Orr, pub. by Sinauer Associates, Sunderland, Mass., 2004. IBSN 0-87893-089-2.

It's worth mentioning that I already have some feedback on this book, thanks to Victor Lambou in the I.B.S. Members list.* Vic points out that the book's authors are strong proponents of the biological species concept, and that they have loosened Mayr's definition somewhat. He also pointed out the additional criterion of "reproductively isolated by substantial but not necessarily complete reproductive isolation" from other populations. Vic also mentions another publication, a short review paper by Loren H. Rieseberg & J.H. Willis, "Plant Speciation" in SCIENCE, vol. 317: pp. 910-914 (2007). This one is worth looking at as well, but full text may only be available to subscribers to SCIENCE. You could perhaps request a reprint from the first author at his e-mail address, <Lriesebe@interchange.ubc.ca>.

I'm indebted to Vic for the feedback. I'll return to this topic as I read my way through the book, and I hope I will have more feedback to relay through this blog as well.

Good gardening,

Jim

* I.B.S. is the International Bulb Society.

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