More About Plant Speciation (a bit off topic)

Joe Shaw
Tue, 10 Jul 2007 15:49:17 PDT
Hi Gang,

There are many unresolved questions in Plant Biology.  One that has always interested me concerns speciation:  when or how do species arise?  The simplest way I think about it is that one species of plants eventually gives rise to two daughter species--how does it happen and what keeps the daughter populations from merging in to one again.  

The problem is that to remain separate species, the daughters have to be prevented from interbreeding; a sterility barrier is generally thought to be needed.  The barrier can be a geographic barrier such as a mountain range or an ocean, or the barrier can be temporal (one daughter blooms in spring and another in summer, and never the pollen shall mix).

The issue is of interest to bulb growers for several reasons:
1.  How do we know when to collect/acquire different but similar plants for our collections,
2.  How can we predict what plants might produce offspring together, and; 
3.  How can we decide if something is a variety, a subspecies, etc.  

One type of sterility barrier is hybrid necrosis.  Hybrid necrosis involves death of cells, death of tissues, reduced growth rate, and other problems.  A typical result of hybrid necrosis is death of an embryo after fertilization.  Thus, pollen is accepted, and a pollen tube is allowed to grow to the egg in the ovary, and fertilization occurs.  But, the developing embryo dies from hybrid necrosis.  

The result is that two plant species are functionally sterile; they produce no offspring.  Of course, hybrid necrosis might only be 90% effective, or 50%, 99%, etc.  Thus, a few progeny might be produced now and then or in the hands of a clever hobbyist.   

The interesting thing about hybrid necrosis is that it is, at least in some cases, related to disease resistance.  Thus, the necrosis might occur with a fungus infects a leaf and a hypersensitive reaction occurs.  The hypersensitive reaction is more than just the fungus killing a few plant cells; hypersensitivity refers to the plant actively killing a zone of cells around an infection site.  The ring of dead cells releases antifungal compounds and can prevent the spread of a fungus.  

Therefore, the hypersensitivity response is generally regarded as a "good thing" for plants, a feature that allows plants to survive infections.  It is pretty easy for a plant to survive the loss of part of a leaf or even a few leaves, as opposed to the infection spreading through the whole plant.  In contrast humans don't tend to drop off a finger when they get a small infection in it.  

Back to speciation and sterility barriers:  defense genes (sometimes) evolve rapidly in plants because pathogens are always evolving and changing (just think of all the varieties of human influenza).  So, plants keep changing their repertoire of defense genes.  When a species separates into two daughter species, it may be that defense genes are some of the earliest "big" changes that accumulate.  Then, when the two daughter species may encounter each other they have different sets of defense genes and they may tend to invoke hybrid necrosis in the new zygote.  A simple way to put it is that the newly fertilized egg may have to immune systems, and each system may cause autoimmunity (immune response directed at self).  

Hybrid necrosis is proposed as a mechansim to keep some species separate once it develops.  I find the whole idea interesting, especially when I consider hotspots of diversity such as the Cape Florisitic Province.   Mountains and deserts may allow a species to split into two daughter species, but they are not separated by 10,000 km, and the daughter species may again encounter each other:  what keeps them from merging back into a single species?  The Cape Floristic Region is tiny, in a geographic sense, but huge in terms of plant diversity.  There are many possible reasons for the phenomenon, but perhaps rapid evolution of plant immune responses is a mechanism that keeps daughter species separate even though they can grow side-by-side.   

Perhaps we all owe a big "thank you" to all the pathogens that attack bulbs and geophytes, in recognition of their help in providing us with diversity.  

LINK 1:  Rapid Evolution and Hybrid Incompatibility…  

LINK 2:  Hybrid Necrosis: autoimmunity as a potential gene-flow barrier in plant species…  

LINK 3:  Cape Floristic Region…  

LINK 4:  Plant Speciation (commentary)…


Conoe TX  

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