Wednesday, October 9, 2013

Beyond Natural Selection –Shortcuts in Plant Evolution

In Darwin’s understanding of evolution, change in the characteristics of organisms occurred very slowly through the painstaking action of natural selection.  In the 20th century such change was defined genetically as the spread of favorable mutations through a population.  That simple view of evolution prevailed until relatively recently, but we now know that there are a number of ways in which dramatic changes in an organism’s genome, or in the expression of that genome, can occur virtually overnight and lead to a dramatic evolutionary change.  This is sometimes referred to as evolutionary saltation (jumping).

One of the simplest, and most familiar examples of overnight change in plants, is the hybridization of crop plants and garden flowers.  Such hybrids, when guided and selected by plant breeders, exhibit the best attributes of the parents and typically great vigor. Hybridization also occurs between related species in nature, and sometimes gives rise to permanent new kinds of plants.  Hybrids are often sterile, because the different chromosome structures of the parents are incompatible and cells cannot undergo meiosis to initiate the reproductive cycle.  However, if the chromosomes of such a hybrid are doubled, either through an accident of cell division, or on purpose by plant breeders, the chromosomes can then pair up and reproduce normally.  Such plants are called polyploids, because they have multiple complete sets of chromosomes.  The earliest form of wheat, emmer wheat, originated as an accidental natural hybridization before it was cultivated by humans.  Seedless bananas are from sterile, triploid plants that also originated from natural hybrids thousands of years ago.

The comparative study of chromosomes has provided extensive evidence of polyploids resulting from both hybridization (allopolyploids) and from duplication of an ordinary genome (autopolyploids) throughout the history of plants.  Such events can jumpstart new evolutionary trends.  Even without the novel combination of traits inherent in hybrids, a simple doubling of chromosomes creates duplicate sets that can accumulate new mutations, and eventually new useful traits.  

Cyanobacteria, such as this Anabaena,
are photosynthetic bacteria that came
about through the combination of
electron transfer chains from two
different sulfur bacteria through
horizontal gene transfer.
Photo by Elapied.
A more technological version of hybridization is the direct transfer of DNA from one organism to another thorugh genetic engineering.  In this way, DNA from unrelated organisms can be combined. Though such genetically modified crops are much maligned, this technology has ancient origins and has been practiced by bacteria for billions of years.  This bacterial form of hybridization is called horizontal gene transfer.  This is the promiscuous practice of bacteria gathering up DNA from other dead bacteria and incorporating it into their own genome.  Though the results might be disastrous at times, this process has resulted in a number of major evolutionary breakthroughs, such as the invention of modern oxygen-releasing photosynthesis.   The complex photosynthetic machinery of cyanobacteria and modern plants combines the abilities of at least two, and probably three distinct bacterial ancestors.   

When more complex eukaryotic organisms evolved – organisms with nuclei and an internal cytoskeleton capable of ingesting other smaller cells – a remarkable series of symbiotic events occurred.  Bacteria capable of aerobic respiration were taken into the larger cells, where they were domesticated and became mitochondria.   Later, cyanobacteria were ingested and gradually modified into chloroplasts.  Here whole organisms are combined into one, not just their DNA.  This process of endosymbiosis resulted in the first photosynthetic eukaryotes, or algae.

Euglenas are protists that relatively recently acquired
chloroplasts through a process akin to the original
endosymbiosic event between an ancient protist and
a cyanobacterium.  In this case, chloroplasts were taken
from green algae fed upon by a carnivorous euglenoid.
The event created a new line of  plant-like organisms.
Photo by Deuterostome (Own work) [CC-BY-SA-3.0
(http://creativecommons.org/licenses/by-sa/3.0)],
via Wikimedia Commons.  
In addition to these various ways of combining the genomes of different organisms, are equally dramatic changes resulting from changes in how genes are expressed.   Every cell in an individual’s body has exactly the same set of genes.  Your left toe, for example, has the genes for making a complete, functional eyeball.  Though it might come in handy when looking for a lost sock under the bed, nature has thought better of it and the genes for eyeball development are turned off during toe development. Developmental of an individual organism involves the turning on and turning off of particular genes in a precisely controlled sequence, sometimes in response to environmental cues.  A system of regulatory (homeotic) genes is responsible for that orderly process, insuring that genes are turned off and on at the right time and in the right place.  Mutations in homeotic genes therefore can have dramatic effects. Diabolical fruit fly geneticists, for example, have been able to tamper with those genetic controls, resulting in eyes, wings, and various other organs popping up where they don’t belong. 

Mutations in key regulatory genes are evident, or at least suspected, in the history of plant life. Some of those have affected the behavior of haploid and diploid cells during reproductive processes.  In animals, the haploid cells that result from meiosis have the specialized shape and behavior of gametes: sperm or egg, which have the single goal of finding one another and combining into one.  The diploid cell (zygote) that results from that union is programmed to start dividing and undergo development into a new individual, though it doesn't have any genes that are absent from the gametes' nuclei.   

Ulva and other forms of algae may have evolved alternation of generations
through mutations that allowed vegetative growth in the diploid zygote that
normally (in most algae) divide directly through meiosis to produce spores.
In this new diploid generation, meiosis and spore-production are delayed
until a full-sized plant identical to the original haploid plant has formed.
Plants differ from animals in that they have two distinct multicellular bodies during their life cycle, one that is diploid and one that is haploid.  Most green algae go about their daily lives as haploid individuals.  Sperm and egg combine, much as they do in animals, but the zygote does not divide to form a new multicellular individual as it would in animals.  Instead, it goes directly into meiosis, usually to produce specialized dispersal cells called zoospores.  Zoospores then settle to produce new haploid individuals, which may be unicellular or multicellular.  Being haploid or diploid determines which developmental genes are turned on, via some key regulatory gene.  In some green algae, such as the sea lettuce (Ulva), however, the zygote, which in other algae is programmed to do nothing more than undergo meiosis, goes through a developmental process identical to that of the haploid plant.  So Ulva populations exist as a mixture of haploid and diploid plants.  The only difference is that the diploid plants (sporophytes) eventually produced zoospores through a much delayed meiosis, and the haploid plants (gametophytes) produce gametes through ordinary cell division.  This alternation of multicellular haploid and diploid generations apparently resulted from a mutation in the regulatory gene that previously prevented the diploid zygote from developing into a multicellular individual.

Something similar appears to have happened in the early land plants, though this is somewhat speculative at this point.  The ancestors of land plants were green
Aglaophyton, an early diploid land plant.
Such plants originated when genes for
indeterminate growth and branching were
turned on in the zygote of normally
haploid plants similar to modern liverworts,
much as occurred in Ulva.
 Drawing by Griensteidl
[GFDL (http://www.gnu.org/copyleft/fdl.html) 
algae that had typical haploid bodies.  While the zygotes of algae go directly into meiosis to produce a handful of spores, those of early land plants delayed meiosis, and instead divided through ordinary division to produce a mass of diploid cells called a sporangium.  Within the sporangium, many cells underwent meiosis, producing a large number of spores.  The direct descendants of plants with this strategy are the bryophytes (mosses, liverworts and hornworts), which have all remained as haploid vegetative plants (gametophytes) alternating with simple, diploid, spore-producing bodies (sporophytes). 

In one ancient land plant, however, a mutation occurred that allowed the diploid zygote to grow like the haploid plant, branching  to ultimately produce many sporangia.  This event was probably much like that occurred in the ancestors of Ulva. These diploid branching plants were the ancestors of the vascular plants (ferns, gymnosperms, and flowering plants).  Their alternate haploid generations shrank over time to become minimal sperm and egg producing structures.  So modern land plants consist of haploid bryophytes and diploid vascular plants.

The most common form of carpel in
flowering plants is  like these in Eranthis,
appearing as leaves folded around
developing seeds (ovules).
Another “giant leap for plantkind” may have come during the evolution of flowers.  It has
Early stamens may have
been flat and leaf-like,
as in this Degeneria
always been a mystery as to where the signature structure of angiosperms - the closed seed chamber known as the carpel - came from.  It has long been assumed to be a leaf-like structure that folded around a series of embryonic seeds (ovules), which were borne directly on the leaf as in ancient seed ferns, or on a branching structure that became enclosed by a leaf.  According to the “mostly male theory” of  Michael Frohlich (2002), however, the first floral organs to form in a flower-like, spiral arrangement were the male structures (stamens).  These were at the time flat and leaf-like.  Ovules were produced elsewhere on the plant, but another genetic accident had the ovule-development genes turn on during stamen development, forming ovules instead of pollen sacs on some of the stamens.  This is comparable to eyeballs forming on toes!  These transsexual stamens then became carpels as they folded around the developing ovules.

More recently,  maize (or corn in America), Zea mays, originated rather suddenly through modification of just a few genes that controlled the location of the female flower spikes, and the nature of the seeds, in a wild plant called Teosinte.
Teosinte, on the left, has very hard grains in simple spikes
located just below the male tassels.  Early forms of maize appeared rather
suddenly with mutations that relocated the cobs lower down
on the stem and that gave the grains a softer, more edible texture.  The center
specimen is a teosinte X maize hybrid that may be similar to early maize.
 Human selection gradually developed the larger cobs we know today.
Photo by John Doebly.

Finally, I must mention the rapidly growing field of epigenetics.  In the broadest definition, epigenetics includes the processes that turn genes on and off to result in different organs, phenotypes, or environmental responses. Genetic changes that enable plants to start producing flowers instead of leaves, for example, are most often triggered by changes in temperature, day length, or other factors.  Epigenetic phenomena can results in rather different looking individuals without any change in their DNA code.  Current focus in epigenetics concerns patterns of gene suppression that adapt individuals to varying environmental conditions, and which sometimes can be passed on to future generations.   Ironically, this resembles the long-discredited Lamarckian idea that characteristics acquired during an individual's lifetime can be passed on to its descendants. For long-term evolutionary trends, true genetic change is required, but heritable epigenetic changes in the short term may allow populations to adapt to changing conditions, after which mutations and ordinary natural selection can reinforce those changes and make them permanent.  The extent to which epigenetics can influence the bigger picture of evolutionary change is still being hotly debated.

In sum,  through the tricks of horizontal gene transfer, endosymbiosis, hybridization,  mutation in regulatory genes, and epigenetics, the evolution of photosynthetic organisms has periodically taken dramatic leaps forward. 

Reference:

Frohlich, M. W. 2002.  The Mostly Male theory of flower origins: summary and update regarding the Jurasssic pteridosperm Pteroma, in Developmental Genetics and Plant Evolution, C. B. Cronk, R. M. Bateman, and J. A. Hawkins, Eds., Taylor and Francis.  London and New York.

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