Wednesday, November 16, 2011

The truth about sex in plants

It may surprise you to know that plants reproduce by sperm and egg, the same as animals. Well not quite the same, of course, since plants can’t move around or physically interact during the process.    This was a huge problem for the first plants to creep up onto dry land.  Their ancestors were green algae, who as a group mostly release their sperm cells (and sometimes their eggs as well) directly into the water to fend for themselves.  In fact many aquatic animals including sea stars and most fish, follow the same strategy.    Releasing sperm and egg cells into dry air, however, just doesn’t work!   
The seed plants eventually solved the problem through the invention of pollen grains -  private limousines  in which  sperm cells ride comfortably through the air. Pollination of pinecones and flowers is what you usually hear presented as the sex life of plants.   But the technology of pollen grains was long in coming.  How did the first plants manage with their sperm cells flopping around on dry land?
Sperm cells were indeed naked and vulnerable as they made their first journeys on land, and they still are in some modern plants like mosses and ferns.   Mosses and ferns are, however, numerous and diverse.  In the sex department, they apparently do just fine, thank you very much.  But how exactly? We’ll focus on ferns for now.  Mosses do things a bit differently, and we’ll return to them in a future post. 
 Like their aquatic ancestors, the sperm cells of ferns must swim through water to find an egg.  This can only take place when the soil is quite wet, and even so, sperm cells can’t travel very far - a few centimeters at best, so suitable mates must be quite close.  However, even if sperm and egg cells were dropped from the fronds of adjacent ferns, any two plants within sperm range of each other would most likely be siblings or at best  cousins, (or at worst  branches of the same plant!) Reproduction could take place only between nearest neighbors, generation after generation.  Plants would exist in pockets of incestuous interbreeding, and everyone knows how bad that would be.  The whole evolutionary purpose of sex is to create genetically diverse populations through breeding among widespread and genetically different individuals.  How did the early land plants escape from this trap?
The solution is something called alternation of generations.  This involves separating the reproductive process into two phases, with a small alternate “individual” as a go-between.  So there is the main plant that we see every day, and a small temporary plant that lives just long enough to complete its reproductive function.   In the fern, the main plant produces not sperm and egg but spores.  These can be seen at certain times of the year, produced in dense clusters of spore chambers (sporangia) on the lower surfaces of the fronds.  The spores are hard and dry and can be dispersed great distances by the wind.  The spores of different individuals, sometimes from widely spaced populations can whirl around in the wind and by chance land together in a suitably moist piece of ground.  
The tiny gametophytes of ferns are
typically little more than 1 cm long
and lie flat against the ground, Sperm
and eggs are produced in tiny
on the undersurface, among with the
root-like rhizoids seen here. from
Haupt, A. W. 1953. Plant Morphology.
McGraw-Hill. New York.
These genetically mixed populations of spores then germinate into the tiny alternate plants that are small enough and close together enough for sperm cells to swim between them.  So it is these alternate plants (the gametophytes) that conduct the sexual phase of reproduction.    In fern gametophytes, sperm cells and eggs are typically produced at slightly different times, so as to avoid the dreaded self-fertilization.  In some species individual gametophytes produce either sperm or eggs, not both – an even better insurance against self-fertilization. 

The large, ordinary plants that we
recognize as ferns are the
sporophytes.  Spores are produced on the
undersides of the fronds.

The big fern plants (the ones we actually see when we walk through the woods) are called sporophytes, because their reproductive task is to produce spores.   Ferns sporophytes have two sets of chromosomes in every cell (they are diploid) like animals, and the special cells on their fronds that produce spores undergo meiosis, a special type of cell division that results in cells with just one set of chromosomes (they are haploid).  So the spores, the gametophytes, and the gametes are all haploid.  When sperm and egg unite, they produce a diploid zygote, which develops into a new diploid sporophyte.   In ferns and seed plants, it is the sporophytes that get large, living often for many years, while the gametophytes are very small and ephemeral.  In mosses and liverworts that situation is reversed, but I’ll get to that in another post.
By separating the reproductive process into two phases, or “generations,” plants overcome their own immobility as well as the limited mobility of their sperm cells, and produce genetically diverse descendants.   Long-distance dispersal and mixing of different genotypes is accomplished by inert spores, and the joining of sperm and egg is then accomplished over a short distance by the tiny gametophytes.   Animals that move about and choose their mates don’t need an intermediary gametophyte.  They produce sperm or egg directly through meiosis, and the males actively deliver the sperm to the females themselves.
But how does all this translate into pollen grains?  Seed plants still employ tiny gametophytes for producing sperm and egg, but these are even tinier and harder to see.  To make a long story short, in the transition from fern-like ancestors to the first seed plants, specialized, sperm-producing gametophytes shrank down into tiny 2- or 3-celled structures that remained within the wall of the spore.   In other words, a pollen grain is a spore that contains a tiny, prepackaged male gametophyte.  The female gametophyte, containing an egg, forms within the embryonic  seed (ovule).  The pollen grain, like spores in general, is dispersed some distance from its parent.  When it lands in the vicinity of an ovule on another plant, a pollen tube develops that carries the sperm cells directly to the egg.   Neither the sperm-producing gametophyte nor the egg-producing gametophyte lives on the ground, and the sperm cells don’t have to swim through wet soil.   This allows seed plants to live and reproduce in a greater variety of habitats than their predecessors.

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