semillas la vida en cápsulas de tiempo - Clh.es

discovery: he demonstrated that the alternation of generations in seed plants follows the same principle as in mossesand ferns and thus proved their evolu-tionary link. To grasp the significance of Hofmeister’s ingenious discoveryand to under-stand how seeds evolved, it is necessary to take a closer look at the private life of seed plants.It’s all about sexJust as human life starts with the union of a sperm from the father and an egg cell from the mother, with eachparent contributing one set of chromosomes, a new plant is created when a male sperm and a female egg cellmeet. In all living beings, the chromosomes contain the genes that determine all the characteristics of theorganism. By mixing the chromosomes – and thus the genetic traits – of two different individuals, a neworganism with a different and perhaps better combination of characteristics is created. It is sex that provides the“raw material” for evolution.Meiosis and the legend of the rice grain on the King’s chessboardThe key to sexual reproduction is a sophisticated way of cell division called reduction division or meiosis. Meiosisis a universal process in all sexually reproducing organisms, both plants and animals. It reduces the number ofchromosomes in the gametes (sperm and egg cells) to half, either directly (in animals) or indirectly (in plants viathe gametophytes). Without it, the number of chromosomes would double in each new generation, like the ricegrain in the famous mathematical folktale from India: a long time ago, a wise man performed a service for theKing of Deccan. The King insisted on rewarding him and, after some hesitation, the humble man asked for asingle grain of rice for the first square on the King’s chessboard, two for the second square on the second day,four for the third square on the third day, and so on. The number of rice grains would be doubled every day untilevery square of the chessboard was filled. The king, who had never heard of exponential growth, agreed, foolishlyas he would soon discover. He owed the man 18 million billion rice grains, or, more precisely, 18 446 744 073709 551 616 (2 to the 64th power) – more rice than could be grown on the entire surface of the Earth includingthe seven seas.The same would happen to the number of chromosomes in subsequent generations if meiosis did notprecede each act of sexual reproduction: for sexual reproduction to function, gametes (egg cells and sperm cells)must be haploid (contain only one set of parental chromosomes) so that they produce a new diploid organism,which contains no more than two sets of chromosomes.Alternating generationsSome plants are capable of vegetative propagation (by producing suckers, like strawberries, for example), whichdoes not create a new kind of individual but simply clones of the mother plant, but most plants reproducesexually. Just like in humans and most animals, a sperm has to fertilise an egg cell to produce the next generation.For green algae – one group of which was the ancestor of our land plants – fertilization was never a greatproblem. Their aquatic lifestyle allowed them to release their sperm cells into the water, where they could swimfreely to find an egg cell; a simple and effective, albeit hugely wasteful method of fertilization.When plants left the water and began to conquer the land, most probably at the end of the Ordovician orat the beginning of the Silurian (about 445 million years ago), they had to adapt to the drier conditions of lifein the open air. Having mobile sperm that require water to swim across to an egg cell to be fertilized was a realdisadvantage when living on land. Today’s spore-producing land plants, such as mosses, clubmosses, horsetails andferns, have yet to solve this problem. Their distribution is limited by the requirements of the egg- and spermproducingphase of their life cycle. This is why they are usually found growing in humid environments or in areaswhere wet periods are common in otherwise dry con-ditions (e.g. xeric ferns like Cheilanthes).Despite this handicap, these spore-producing plants possess an effective method of sexual reproductioninvolving a cycle called alternation of generations, 2 something not found in animals. Alternation of generationsmeans that their life cycle involves two generations, a diploid generation (with two sets of chromosomes, one setfrom each parent) and a haploid generation (with only half the number of chromosomes), each of which canonly give rise to the other. And this is where the difference between seeds and spores becomes obvious. Seedsgerminate to produce a new diploid generation, whereas spores produce a haploid generation. This may soundcomplicated but a comparison of the life cycles of the different types of land plants will clarify this fundamentaldifference between seeds and spores starting with the most primitive land plants, which are mosses.The life cycle of mossesMoss spores give rise to the haploid generation, the small familiar moss plants. When they reach maturity, theyproduce both male organs (antheridia) that release mobile sperm, and female organs (archegonia) containing eggcells. Sperm and egg cells are generally called gametes, which is why the small moss plant that produces them isalso referred to as the gametophytic generation or gametophyte (literally gamete plant).A romantic swimIn the presence of water (rain, dew, spray from a river or waterfall), the sperm cells are released from theantheridia and swim across to the egg cells waiting for them in the archegonia of either the same or aneighbouring plant. Like the gametophytes which produced them, the sperm and the egg cell are both haploidand contain only one set of chromosomes. After fertilization the egg cell contains two sets of chromosomes (i.e.,it becomes diploid) and is called a zygote. The zygote stays on the mother plant, which provides it with waterand nutrients, and as it develops it will produce a tiny moss embryo. This embryo grows into the familiar mosscapsule that represents the diploid generation of the moss life cycle. Since new haploid spores are produced insidethe moss capsule, it is also called the sporophytic generation or simply sporophyte (literally spore plant). When thecapsule is ripe, it opens at the top and releases the spores to be blown away by the wind or washed away by water.In a suitable location, the spores grow into a new moss plant (gametophyte) and the reproductive cycle startsanew.The life cycle of fernsIn principle at least, ferns have the same sex life as mosses. In the right place and with enough moisture, a fern sporewill produce the gametophytic generation. However, the gametophyte of a fern does not look like a fern at all. Itis a small, green, flat lobe very similar to the gametophyte of some liverwort. This haploid plantlet, also called aprothallus or prothallium (literally pre-shoot), usually produces its sex organs (antheridia and archegonia) on itsunderside to prevent them from drying out. Although fertilization in ferns follows the same pattern as in mosses,what happens afterwards puts every moss to shame: the zygote of a fern does not just produce a simple small capsuleon a stalk that needs the support of the mother plant. Instead, it develops into a totally independent, impressive andoften very beautiful plant that can grow for many years, sometimes becoming the size of a tree. This means that inthe life cycle of ferns, the gametophytic generation remains relatively poorly developed whereas the sporophyticgeneration is strongly enhanced. 3 Hence, every fern plant we see is a sporophyte. There are often regular rows ofsmall, slightly raised, brown spots on the underside of fern fronds. These brown spots, called sori (singular sorus), aregroups of spore containers (sporangia; singular sporangium) covered by a protective umbrella, the indusium. Like mossspores, fern spores are minute and float easily in the air, sometimes travelling for miles on a light breeze.What does the sporophyte have that the gametophyte does not?A life cycle favouring the sporophyte has a clear evolutionary advantage that lies in the genetics of the plant: thesporophyte is diploid and thus has two copies of each gene. This means that if one gene malfunctions owing toa mutation, the plant has a second copy of the same gene, which acts as a back-up and compensates for thedamage. Genetic mutations are therefore less likely to have a negative impact on the vitality of a sporophyte thanon the vitality of a gametophyte. Moreover, two slightly different copies of the same gene allow the sporophyteto respond more flexibly to changes in the environment resulting in better fitness than in the gametophyte.When males are micro and females are megaWhile most mosses are monoecious, meaning they bear antheridia and archegonia on the same gametophyte, someare dioecious, meaning they produce sperm and egg cells on different individuals. Most ferns prefer the former.Only a very small group of ferns, the water-ferns to which the water clover (Marsilea, Regnellidium), the pillwort(Pilularia), the duckweed fern (Azolla) and the floating fern (Salvinia) belong, produce separate male and femalegametophytes. The sporophyte of these ferns must therefore produce two kinds of spores: male and female. Apartfrom their gender, male and female spores also differ in size, a condition called heterospory. Because of thisdifference in size, the male spores are called microspores and the female spores megaspores. Microspores give rise tomale microgametophytes and megaspores to female megagametophytes. The containers in which these two types ofspore are produced on the sporophyte are microsporangia and megasporangia. The leaves of the sporophyte whichbear these micro- and megasporangia are called micro- and mega-sporophylls, respectively. This avalanche oftechnical terms may be daunting but it makes comparisons of the different life cycles of plants much easier. Fornow, it is enough to remember that micro means male and mega means female.In present-day water-ferns, heterospory almost certainly represents an adaptation to the aquatic lifestyle towhich they reverted. Nevertheless, it was the preferred condition among the ancestors of our seed plants. In fact,as will soon become clear, heterospory played an important role in the evolution of seeds.How seeds evolvedThe first seed plants or spermatophytes appeared some 360 million years ago, towards the end of the Devonian.These early seed plants combined seeds with fern-like foliage and were thought to be intermediate between fernsand modern seed plants, hence the name seed ferns or pteridosperms. However, it is now known that fernsthemselves were not the direct ancestors of seed plants. This role falls to an extinct, rather obscure side branch ofhetero-sporous fern-like plants. The exact events and transitional stages that led from heterosporous fern-like266 Semillas – La vida en cápsulas de tiempo