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Wednesday, September 16, 2009

EVOLUTION OF FLOWERS




Flowers represent the most advanced stage of evolution of plants. As in humans the aim of the emergence of flowers is to enable a broad mix of genes so that the resulting variety, allow infinite reproduction of the same. As Darwin did not know the existence of genes, the emergence of flowers was for him a great mystery. Today, genetic studies are shedding light on these enigmas. I) We know now that emergence of flowers was achieved recycling old genes and that the closest relatives of flowering plants are plants without flowers, that produces seeds (pine, gingko). The oldest living flowering plant, Amborella trichopoda is a shrub found in the island of New Caledonia, dating 136 million years.

II) Flowers began to be understood with the discovery in 1980 of the first genes that guided their development. At that time, artificial mutations in Arabidopsis resulted in grotesque changes: petals that grow where should be stamens, sepals instead of leaves. In Arabidopsis, a gene called AP3 is required to build petals and stamens. Soon it became known that flowers originated from a basic set of genes, appropriate to geography. III) The final evolution of the flowers led to male and female sex organs on the same axis. When a pollen grain fertilizes an egg, it provides 2 sets of DNA. One grain is destined to the sac that surrounds the egg, filled with endosperm that turn an egg into a seed. In archaic flowers, the endosperm contains a set of genes from male and female. IV) Shortly thereafter, the flowers suffer a revolutionary change incorporating an extra set of genes in the endosperm, which then could store more protein. In roses the strategy involves a duplication of nuclei producing more complex and major changes in the flowers, so that flowers attract more pollinators and frugivores, to disperse their seeds and flowers and stay !alive!.

EVOLUCION DE LAS FLORES

Las flores representan la etapa más avanzada de la evolución de las plantas. Al igual que en los humanos el objetivo de la emergencia de las flores es permitir una mezcla amplia de genes de modo tal que la variedad resultante, permita una reproducción infinita de las mismas. Como Darwin no conocía la existencia de genes, la emergencia de las flores fué para él un gran misterio. Hoy, los estudios genéticos están aclarando los supuestos enigmas. I) Hoy se sabe que la emergencia de las flores se logró reciclando viejos genes y que los parientes más cercanos de las plantas con flores son plantas sin flores, productoras de semillas (pinos, gingkos). La planta con flores viva, más antigua es Amborella trichopoda, un arbusto encontrado en la isla de New Caledonia, con una antigüedad de 136 millones de años.

II) Las flores empezaron a ser comprendidas al descubrirse en 1980, los primeros genes que guíaban su desarrollo. Por esa época, las mutaciones artificiales en Arabidopsis dieron lugar a cambios grotescos: pétalos que crecían donde debían existir estambres, sépalos en vez de hojas. En Arabidopsis, un gene llamado AP3 es requerido para construir pétalos y estambres. Pronto se supo que las flores se originaron a partir de un set básico de genes, adecuados a la geografía. III) La evolución final de las flores las condujo a tener órganos sexuales masculinos y femeninos en el mismo eje. Cuando un grano de polen fertiliza un óvulo, proporciona 2 sets de DNA. Otro grano está destinado al saco que rodea al óvulo, lleno de endosperma que permite convertir un óvulo en una semilla. En flores arcaicas, el endosperma contenía un solo set de genes procedentes del macho y la hembra. IV) Poco después, las flores sufrirían un cambio revolucionario trás incorporar un set extra de genes en el endosperma, que entonces pudo almacenar mas proteínas. En las rosas la estrategia involucra una duplicación de núcleos que produce mayor complejidad y cambios mayores en las flores. Todo esto para que las flores atraigan mas polinadores y frugívoros para dispersar sus flores y semillas y ¡seguir vivas!.

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