semillas la vida en cápsulas de tiempo - Clh.es

ovules. This is mainly the case where the ovary contains only a single erect ovule whose micropyle faces or eventouches the trans-mitting tissue (e.g. in Juglandaceae, Piperaceae, Polygonaceae, Myricaceae, Urticaceae), makingit unnecessary to turn the ovule round.Precocious pubertyEvents inside the nucellus of the angiosperms are initially the same as those in the gymnosperms. Within thetissue of the nucellus, one cell, the megaspore mother cell, undergoes meiosis to produce four haploid megasporesof which three (those closest to the micropyle) usually die. 10 The survivor is called the functional megaspore.From this point on, however, everything is different in angiosperms and – most importantly – much faster.Instead of producing a female gametophyte that consists of thousands of cells and one or more archegonia, themegagametophyte of the angiosperms is simplified. After only three mitotic (normal) divisions of its nucleus, thefunctional megaspore gives rise to eight free haploid nuclei, which are arranged in two groups of four at eitherpole of the megaspore. In the next stage, one nucleus from each group migrates to the centre. As they come fromopposite poles, they are appropriately called polar nuclei. After the formation of cell walls, the mature gametophytetypically consists of eight nuclei distributed over just seven cells, one of which functions as an egg cell. 11 Theseseven cells of the megagametophyte, in angiosperms also called the embryo sac, are arranged in a particular pattern:there are three small cells at the micropylar end forming the egg apparatus, which consists of the egg cell and twoaccompanying synergids. The three cells of the egg apparatus face another set of three antipodal cells at the oppositeend of the embryo sac. Finally, in between the egg apparatus and the antipodal cells is a large central cell containingthe two polar nuclei. Once the female gametophyte has reached this stage, it is ready for fertilization.The progressive reduction of the female (and male) gametophytes observed in seed plants is an impressiveexample of a recurring phenomenon in the evolution of both plants and animals. It is called progenesis. Progenesisis the politically correct term for something very similar to “precocious puberty” and refers to an organism thatreaches sexual maturity while still in its juvenile stage. Certain amphibians and insects are good examples.When it comes to sex, angiosperms want it allThe evolution of the seven-celled/eight-nucleate megagametophyte is new and radical, but it is only the foreplayto a most sophisticated act of sexual reproduction. To begin with, gymnosperms have nothing that wouldcompare to the antipodal cells and the polar nuclei of the angiosperms. The antipodal cells usually have noparticular function and soon degenerate but the polar nuclei assume a unique role: they join the unification ofmale and female. But in what way and why are they allowed to join in this most private affair?On the male side, the microspore starts off as a single cell. Soon, the young microspore undergoes a mitoticdivision inside the pollen grain, resulting in a vegetative cell and a generative cell. Even before the pollen grainsare released from the pollen sacs, the generative cell often divides mitotically to produce the two sperm nuclei.The sperm nuclei are gametes with only a small volume of cytoplasm around the nucleus and no flagellae. Whena pollen grain reaches the stigma of a flower, it germinates with a pollen tube. Its growth directed by the nucleusof the vegetative cell, the pollen tube penetrates the stigma and traverses the style, finally entering the cavity ofthe ovary and eventually the micropyle of the ovule.Upon entering the micropyle, the pollen tube releases the two sperm nuclei into one of the synergids nextto the egg cell. Shortly before this, the two haploid polar nuclei in the central cell have fused into one diploidnucleus and migrated to a position close to the egg apparatus. What happens next is unique in the plant kingdom.Once pollinated, twice fertilisedWhilst their old-fashioned gymnosperm brethren use only one of the two sperm nuclei to fertilise an egg celland allow the other one to either go to waste or fertilise a neighbouring archegonium, angiosperms want both– to achieve an extraordinary double fertilization: one of the sperm nuclei enters the egg cell as usual, while theother one moves down into the central cell where it meets the diploid nucleus (formed by the two polar nuclei)already waiting close by. Then, in the same way as the first haploid sperm nucleus fuses with the haploid nucleusof the egg cell to form the diploid nucleus of the zygote, the second haploid sperm nucleus fuses with the diploidnucleus of the central cell to form a triploid nucleus. Only when this double fertilization has been successful,will the zygote give rise to the embryo and the ovule start developing into the seed. Before taking a closer lookat the development of the embryo, the fate of the enigmatic triploid central cell should be explored.The triploid lunch pack of the angiospermsEven before the zygote produces a recognizable embryo, the triploid central cell grows into a tissue whichconstitutes the defining component of the angiosperm seed, the endosperm. 12 The endosperm surrounds thegrowing embryo and nourishes it during its development. As the seed matures, the embryo grows deeper into theendosperm. What the embryo has not consumed by the time the seed is mature persists in the seed and serves asthe young sporophyte’s food reserve during germination. Since it forms the main constituent of cereal grains,endosperm is the reason that rice, wheat, maize, oat and millet provide the staple food for billions of peopleworldwide. It is this unique and unparalleled triploid seed storage tissue that more than anything else characterisesthe angiosperms. But why does it emerge from a second fertilization when it does not produce an embryo?Is the endosperm a sacrificial twin?In 1898-99 the Russian biologist Sergei Gavrilovich Navashin (1857-1930) and the French botanist Jean-Louis-Léon Guignard (1852-1928) discovered – independently – the develop-mental origin of endosperm from doublefertilization. But since then, the evolution of the specific events that led to the formation of a triploid endospermhas remained a mystery.Recent research has shown that members of the gymnospermous Gnetales – believed by some to be theclosest living relatives of the angiosperms – also undergo a process of double fertilization. In Gnetales, however,double fertilization does not lead to the formation of a zygote and an endosperm but – as would be expected –to two diploid zygotes. One theory is that in the long extinct ancestors of the angiosperms, the secondfertilization event yielded a genetically identical twin embryo but once the angiosperm stem lineage branchedoff, this twin embryo evolved into an embryo-nourishing structure, the endosperm.However, it is still not proven that the Gnetales are the closest living relatives of the angiosperms. Recentevidence suggests that they may be more closely related to pines. Their method of sexual reproduction may not,therefore, be of immediate relevance to under-standing how double fertilization evolved in angiosperms.Whether scientists will ever be able to solve this riddle is uncertain. Because of incomplete fossil records,angiosperms seem to have appeared suddenly and with considerable diversity in the Earth’s history but withoutobvious antecedents. The evolutionary origin of the angiosperms – the most prominent unresolved issue in plantevolutionary biology – therefore remains Darwin’s “abominable mystery” of more than a century ago.The advantages of double fertilizationJust as the evolutionary origin of the endosperm remains a mystery, the advantages of double fertilization are notcompletely clear. By incorporating both maternal and paternal genes, the endosperm tissue becomes geneticallyidentical to the embryo. This could enhance the development of the embryo by reducing the risk of geneticincompatibilities between mother and offspring. This is also a possibility in gymnosperms, where the tissuesurrounding the embryo (the haploid megagametophyte) is entirely maternal.From an economic point of view also, the evolution of endosperm seems to be an improvement. Althoughgymnosperms such as ginkgos, cycads and conifers can be proud of their seeds, the way they “make” them is notparticularly efficient. Their primeval sex life forces them to produce their storage tissue (the massivemegagametophyte) in advance, before the egg cell is fertilised. Angiosperms put their energy into the formationof the expensive, energy-rich endosperm only after successful fertilization of the ovule. If the pollination of aflower fails, angiosperms have not wasted too much precious energy and materials. Conservation is always a greatadvantage in the evolutionary race.The embryo of the angiospermsAfter fertilization, or sometimes even before, the synergids and antipodal cells degenerate while the zygote startsto divide mitotically. In all angiosperms, the very first division of the zygote is asymmetrical. It divides the zygotetransversely into a large basal cell facing the micropyle, and a small apical cell facing in the opposite direction.This first division deter-mines the polarity of the embryo. The apical cell grows into the embryo proper whereasthe larger basal cell produces a stalk-like suspensor, similar to the one already encountered in gymnosperms.Originally, the suspensor was merely conceived as a means to anchor the embryo at the micropyle while itpushed the embryo deeper into the endosperm. Today, we know that its function is much more complex. Thesuspensor not only nourishes the young sporophyte with nutrients transferred from the mother plant but alsocontrols the early stages of development of the embryo by supplying it with hormones. Unlike the embryo, thesuspensor is short lived. In the mature seed it has long since disappeared, often without trace.Monocots and DicotsInitially the embryo proper is just a globular lump of cells but it soon starts to differen-tiate into the embryonicaxis (hypocotyl), with the root (radicle) at one end and the first leaves of the young sporophyte, the seed leaves orcotyledons, at the other. Since the development of the embryo starts right underneath the micropyle, the tip ofthe radicle always marks the spot.The embryo of gymnosperms can have any number between one and more than ten cotyledons, but theembryo of angiosperms has either two or just one. Botanists have long used this very convenient, clear-cutdistinction to separate the angiosperms into two groups, the Dicotyledons with two seed leaves and theMonocotyledons with only one seed leaf. There are very few exceptions to this rule where the number ofcotyledons exceeds two. Some individuals of Magnolia grandiflora (Magnoliaceae), for example, may occasionally272 Semillas – La vida en cápsulas de tiempo