En biología , un híbrido es la descendencia resultante de combinar las cualidades de dos organismos de diferentes razas, variedades, especies o géneros a través de la reproducción sexual . Los híbridos no siempre son intermedios entre sus padres (como en la herencia de mezcla ), pero pueden mostrar vigor híbrido , a veces creciendo más o más que cualquiera de los padres. El concepto de híbrido se interpreta de manera diferente en la cría de animales y plantas, donde existe interés en la filiación individual. En genética , la atención se centra en la cantidad de cromosomas . En taxonomía, una cuestión clave es qué tan estrechamente relacionadas están las especies parentales .
![](http://wikiimg.tojsiabtv.com/wikipedia/commons/thumb/e/e0/Juancito.jpg/440px-Juancito.jpg)
Las especies se aíslan reproductivamente por fuertes barreras a la hibridación, que incluyen diferencias genéticas y morfológicas, diferentes tiempos de fertilidad, comportamientos y señales de apareamiento y rechazo fisiológico de los espermatozoides o del embrión en desarrollo. Algunas actúan antes de la fecundación y otras después. Existen barreras similares en las plantas, con diferencias en los tiempos de floración, los vectores de polen, la inhibición del crecimiento del tubo polínico, la esterilidad somatoplástica, la esterilidad masculina citoplásmica-génica y la estructura de los cromosomas. Sin embargo, algunas especies animales y muchas especies vegetales son el resultado de la especiación híbrida , incluidas importantes plantas de cultivo como el trigo , donde se ha duplicado el número de cromosomas.
El impacto humano en el medio ambiente ha resultado en un aumento en el mestizaje entre especies regionales, y la proliferación de especies introducidas en todo el mundo también ha resultado en un aumento en la hibridación. Esta mezcla genética puede amenazar a muchas especies con la extinción, mientras que la erosión genética del monocultivo en plantas de cultivo puede estar dañando el acervo genético de muchas especies para la reproducción futura. Una forma de hibridación a menudo intencional mediada por humanos es el cruce de especies silvestres y domesticadas. Esto es común tanto en la horticultura tradicional como en la agricultura moderna ; Se han producido por hibridación muchas frutas, flores, hierbas de jardín y árboles de utilidad comercial. Una de esas flores, Oenothera lamarckiana , fue fundamental para las primeras investigaciones genéticas sobre el mutacionismo y la poliploidía. También se realiza más ocasionalmente en el comercio de ganado y mascotas; algunos híbridos salvajes × domésticos bien conocidos son el beefalo y los perros lobo . La cría selectiva humana de animales y plantas domesticados ha dado lugar al desarrollo de razas distintas (normalmente denominadas cultivares en referencia a las plantas); los cruces entre ellos (sin ningún stock silvestre ) a veces también se denominan de manera imprecisa "híbridos".
Los humanos híbridos existieron en la prehistoria. Por ejemplo, se cree que los neandertales y los humanos anatómicamente modernos se cruzaron hace tan solo 40.000 años.
Los híbridos mitológicos aparecen en la cultura humana en formas tan diversas como el Minotauro , mezclas de animales, humanos y bestias míticas como centauros y esfinges , y los Nephilim de los apócrifos bíblicos descritos como los hijos malvados de ángeles caídos y mujeres atractivas.
Etimología
![](http://wikiimg.tojsiabtv.com/wikipedia/commons/thumb/4/4e/Liger.jpg/440px-Liger.jpg)
El término híbrido se deriva del latín hybrida , utilizado para cruces como de una cerda domesticada y un jabalí. El término entró en uso popular en inglés en el siglo XIX, aunque se han encontrado ejemplos de su uso desde principios del siglo XVII. [1] híbridos conspicuos se denominan popularmente con palabras-valija , a partir de la década de 1920 con la cría de híbridos tigre-león ( liger y Tigon ). [2]
Visto por diferentes disciplinas
Cría de animales y plantas
Desde el punto de vista de los criadores de animales y plantas, existen varios tipos de híbridos formados a partir de cruces dentro de una especie, como entre diferentes razas . [3] Los híbridos cruzados simples son el resultado del cruce entre dos organismos reproductores verdaderos que producen un híbrido F1 (primera generación filial). El cruce entre dos líneas homocigotas diferentes produce un híbrido F1 que es heterocigoto ; que tiene dos alelos, uno contribuido por cada padre y típicamente uno es dominante y el otro recesivo . Por lo general, la generación F1 también es fenotípicamente homogénea y produce descendientes que son todos similares entre sí. [4] Los híbridos cruzados dobles resultan del cruce entre dos híbridos F1 diferentes (es decir, hay cuatro abuelos no emparentados). [5] Los híbridos cruzados de tres vías resultan del cruce entre un híbrido F1 y una línea endogámica. Los híbridos cruzados triples resultan del cruce de dos híbridos cruzados de tres vías diferentes. [6] Los híbridos top cross (o "topcross") resultan del cruce de un macho de raza pura o de calidad superior y una hembra de calidad inferior, con la intención de mejorar la calidad de la descendencia, en promedio. [7]
Los híbridos de poblaciones resultan del cruce de plantas o animales en una población con los de otra población. Estos incluyen híbridos interespecíficos o cruces entre diferentes razas. [8]
En horticultura , el término híbrido estable se utiliza para describir una planta anual que, si se cultiva y se cría en un pequeño monocultivo sin polen externo (por ejemplo, un invernadero con filtro de aire) produce descendencia que es "fiel al tipo" con respecto al fenotipo. ; es decir, un verdadero organismo reproductor. [9]
Biogeografia
La hibridación puede ocurrir en las zonas híbridas donde se superponen los rangos geográficos de especies, subespecies o distintos linajes genéticos. Por ejemplo, la mariposa Limenitis arthemis tiene dos subespecies principales en América del Norte, L. a. arthemis (el almirante blanco) y L. a. astyanax (el morado con manchas rojas). El almirante blanco tiene una banda blanca brillante en sus alas, mientras que el morado con manchas rojas tiene tonos azul verdosos más fríos. La hibridación se produce entre un área estrecha en Nueva Inglaterra, el sur de Ontario y los Grandes Lagos, la "región de sutura". Es en estas regiones donde se formaron las subespecies. [10] Se han formado otras zonas híbridas entre las especies descritas de plantas y animales.
Genética
Desde el punto de vista de la genética, se pueden distinguir varios tipos diferentes de híbridos. [11] [12] Un híbrido genético lleva dos alelos diferentes del mismo gen , donde, por ejemplo, un alelo puede codificar un color de pelaje más claro que el otro. [11] [12] Un híbrido estructural resulta de la fusión de gametos que tienen una estructura diferente en al menos un cromosoma, como resultado de anomalías estructurales . [11] [12] Un híbrido numérico resulta de la fusión de gametos que tienen diferentes números de cromosomas haploides . [11] [12] Se produce un híbrido permanente cuando solo se presenta el genotipo heterocigoto , como en Oenothera lamarckiana , [13] porque todas las combinaciones homocigotas son letales. [11] [12] En la historia temprana de la genética, Hugo de Vries supuso que estos eran causados por mutaciones . [14] [15]
Taxonomía
Desde el punto de vista de la taxonomía , los híbridos difieren según su ascendencia. Los híbridos entre diferentes subespecies (como entre el perro y el lobo euroasiático ) se denominan híbridos intraespecíficos. [16] Los híbridos interespecíficos son la descendencia del apareamiento entre especies ; [17] estos a veces dan como resultado una especiación híbrida. [18] Los híbridos intergenéricos resultan de apareamientos entre diferentes géneros, como entre ovejas y cabras . [19] Los híbridos interfamiliares, como entre pollos y gallinas de Guinea o faisanes , se describen de manera confiable pero son extremadamente raros. [20] Los híbridos interordinales (entre diferentes órdenes) son pocos, pero se han elaborado con el erizo de mar Strongylocentrotus purpuratus (hembra) y el dólar de arena Dendraster excentricus (macho). [21]
Biología
Expresión de rasgos parentales
Cuando dos tipos distintos de organismos se reproducen entre sí, los híbridos resultantes suelen tener rasgos intermedios (p. Ej., Uno de los padres de una planta tiene flores rojas, el otro tiene flores blancas y el híbrido, flores rosadas). [22] Comúnmente, los híbridos también combinan rasgos que se ven solo por separado en uno de los padres o en el otro (por ejemplo, un híbrido de ave puede combinar la cabeza amarilla de uno de los padres con el vientre naranja del otro). [22]
Mecanismos de aislamiento reproductivo
Los híbridos interespecíficos se obtienen mediante el apareamiento de individuos de dos especies, normalmente del mismo género. La descendencia muestra rasgos y características de ambos padres, pero a menudo son estériles , lo que impide el flujo de genes entre las especies. [23] La esterilidad a menudo se atribuye al diferente número de cromosomas entre las dos especies. Por ejemplo, los burros tienen 62 cromosomas , los caballos tienen 64 cromosomas y las mulas o burdéganos tienen 63 cromosomas. Los mulos, burdéganos y otros híbridos interespecíficos normalmente estériles no pueden producir gametos viables, porque las diferencias en la estructura cromosómica impiden el apareamiento y la segregación apropiados durante la meiosis , la meiosis se interrumpe y no se forman espermatozoides ni óvulos viables. Sin embargo, se ha informado de fertilidad en mulas con un burro como padre. [24]
Una variedad de mecanismos limitan el éxito de la hibridación, incluida la gran diferencia genética entre la mayoría de las especies. Las barreras incluyen diferencias morfológicas, diferentes tiempos de fertilidad, comportamientos y señales de apareamiento y rechazo fisiológico de los espermatozoides o del embrión en desarrollo. Algunos actúan antes de la fertilización; otros después de él. [25] [26] [27] [28]
En las plantas, algunas barreras para la hibridación incluyen diferencias en el período de floración, diferentes vectores polinizadores, inhibición del crecimiento del tubo polínico, esterilidad somatoplástica, esterilidad masculina citoplásmica-génica y diferencias estructurales de los cromosomas. [29]
Especiación
A few animal species are the result of hybridization. The Lonicera fly is a natural hybrid. The American red wolf appears to be a hybrid of the gray wolf and the coyote,[31] although its taxonomic status has been a subject of controversy.[32][33][34] The European edible frog is a semi-permanent hybrid between pool frogs and marsh frogs; its population requires the continued presence of at least one of the parent species.[35] Cave paintings indicate that the European bison is a natural hybrid of the aurochs and the steppe bison.[36][37]
Plant hybridization is more commonplace compared to animal hybridization. Many crop species are hybrids, including notably the polyploid wheats: some have four sets of chromosomes (tetraploid) or six (hexaploid), while other wheat species have (like most eukaryotic organisms) two sets (diploid), so hybridization events likely involved the doubling of chromosome sets, causing immediate genetic isolation.[38]
Hybridization may be important in speciation in some plant groups. However, homoploid hybrid speciation (not increasing the number of sets of chromosomes) may be rare: by 1997, only 8 natural examples had been fully described. Experimental studies suggest that hybridization offers a rapid route to speciation, a prediction confirmed by the fact that early generation hybrids and ancient hybrid species have matching genomes, meaning that once hybridization has occurred, the new hybrid genome can remain stable.[39]
Many hybrid zones are known where the ranges of two species meet, and hybrids are continually produced in great numbers. These hybrid zones are useful as biological model systems for studying the mechanisms of speciation. Recently DNA analysis of a bear shot by a hunter in the North West Territories confirmed the existence of naturally-occurring and fertile grizzly–polar bear hybrids.[40]
Hybrid vigour
Hybridization between reproductively isolated species often results in hybrid offspring with lower fitness than either parental. However, hybrids are not, as might be expected, always intermediate between their parents (as if there were blending inheritance), but are sometimes stronger or perform better than either parental lineage or variety, a phenomenon called heterosis, hybrid vigour, or heterozygote advantage. This is most common with plant hybrids.[41] A transgressive phenotype is a phenotype that displays more extreme characteristics than either of the parent lines.[42] Plant breeders use several techniques to produce hybrids, including line breeding and the formation of complex hybrids. An economically important example is hybrid maize (corn), which provides a considerable seed yield advantage over open pollinated varieties. Hybrid seed dominates the commercial maize seed market in the United States, Canada and many other major maize-producing countries.[43]
In a hybrid, any trait that falls outside the range of parental variation (and is thus not simply intermediate between its parents) is considered heterotic. Positive heterosis produces more robust hybrids, they might be stronger or bigger; while the term negative heterosis refers to weaker or smaller hybrids.[44] Heterosis is common in both animal and plant hybrids. For example, hybrids between a lion and a tigress ("ligers") are much larger than either of the two progenitors, while "tigons" (lioness × tiger) are smaller. Similarly, the hybrids between the common pheasant (Phasianus colchicus) and domestic fowl (Gallus gallus) are larger than either of their parents, as are those produced between the common pheasant and hen golden pheasant (Chrysolophus pictus).[45] Spurs are absent in hybrids of the former type, although present in both parents.[46]
Influencia humana
Anthropogenic hybridization
Hybridization is greatly influenced by human impact on the environment,[47] through effects such as habitat fragmentation and species introductions.[48] Such impacts make it difficult to conserve the genetics of populations undergoing introgressive hybridization. Humans have introduced species worldwide to environments for a long time, both intentionally for purposes such as biological control, and unintentionally, as with accidental escapes of individuals. Introductions can drastically affect populations, including through hybridization.[12][49]
Management
There is a kind of continuum with three semi-distinct categories dealing with anthropogenic hybridization: hybridization without introgression, hybridization with widespread introgression (backcrossing with one of the parent species), and hybrid swarms (highly variable populations with much interbreeding as well as backcrossing with the parent species). Depending on where a population falls along this continuum, the management plans for that population will change. Hybridization is currently an area of great discussion within wildlife management and habitat management. Global climate change is creating other changes such as difference in population distributions which are indirect causes for an increase in anthropogenic hybridization.[47]
Conservationists disagree on when is the proper time to give up on a population that is becoming a hybrid swarm, or to try and save the still existing pure individuals. Once a population becomes a complete mixture, the goal becomes to conserve those hybrids to avoid their loss. Conservationists treat each case on its merits, depending on detecting hybrids within the population. It is nearly impossible to formulate a uniform hybridization policy, because hybridization can occur beneficially when it occurs "naturally", and when hybrid swarms are the only remaining evidence of prior species, they need to be conserved as well.[47]
Genetic mixing and extinction
Regionally developed ecotypes can be threatened with extinction when new alleles or genes are introduced that alter that ecotype. This is sometimes called genetic mixing.[50] Hybridization and introgression, which can happen in natural and hybrid populations, of new genetic material can lead to the replacement of local genotypes if the hybrids are more fit and have breeding advantages over the indigenous ecotype or species. These hybridization events can result from the introduction of non-native genotypes by humans or through habitat modification, bringing previously isolated species into contact. Genetic mixing can be especially detrimental for rare species in isolated habitats, ultimately affecting the population to such a degree that none of the originally genetically distinct population remains.[51][52]
Effect on biodiversity and food security
In agriculture and animal husbandry, the Green Revolution's use of conventional hybridization increased yields by breeding "high-yielding varieties". The replacement of locally indigenous breeds, compounded with unintentional cross-pollination and crossbreeding (genetic mixing), has reduced the gene pools of various wild and indigenous breeds resulting in the loss of genetic diversity.[54] Since the indigenous breeds are often well-adapted to local extremes in climate and have immunity to local pathogens, this can be a significant genetic erosion of the gene pool for future breeding. Therefore, commercial plant geneticists strive to breed "widely adapted" cultivars to counteract this tendency.[55]
En diferentes taxones
In animals
Mammals
Familiar examples of equid hybrids are the mule, a cross between a female horse and a male donkey, and the hinny, a cross between a female donkey and a male horse. Pairs of complementary types like the mule and hinny are called reciprocal hybrids.[56] Among many other mammal crosses are hybrid camels, crosses between a bactrian camel and a dromedary.[57] There are many examples of felid hybrids, including the liger.
The first known instance of hybrid speciation in marine mammals was discovered in 2014. The clymene dolphin (Stenella clymene) is a hybrid of two Atlantic species, the spinner and striped dolphins.[58] In 2019, scientists confirmed that a skull found 30 years earlier was a hybrid between the beluga whale and narwhal; dubbed the narluga.[59]
Birds
Cagebird breeders sometimes breed bird hybrids known as mules between species of finch, such as goldfinch × canary.[60]
Amphibians
Among amphibians, Japanese giant salamanders and Chinese giant salamanders have created hybrids that threaten the survival of Japanese giant salamanders because of competition for similar resources in Japan.[61]
Fish
Among fish, a group of about fifty natural hybrids between Australian blacktip shark and the larger common blacktip shark was found by Australia's eastern coast in 2012.[62]
Russian sturgeon and American paddlefish were hybridized in captivity when sperm from the paddlefish and eggs from the sturgeon were combined, unexpectedly resulting in viable offspring. This hybrid is called a sturddlefish.[63][64]
Invertebrates
Among insects, so-called killer bees were accidentally created during an attempt to breed a strain of bees that would both produce more honey and be better adapted to tropical conditions. It was done by crossing a European honey bee and an African bee.[65]
The Colias eurytheme and C. philodice butterflies have retained enough genetic compatibility to produce viable hybrid offspring.[66] Hybrid speciation may have produced the diverse Heliconius butterflies,[67] but that is disputed.[68]
A "zonkey", a zebra/donkey hybrid
A "jaglion", a jaguar/lion hybrid
A domestic canary/goldfinch hybrid
In plants
Plant species hybridize more readily than animal species, and the resulting hybrids are fertile more often. Many plant species are the result of hybridization, combined with polyploidy, which duplicates the chromosomes. Chromosome duplication allows orderly meiosis and so viable seed can be produced.[69]
Plant hybrids are generally given names that include an "×" (not in italics), such as Platanus × acerifolia for the London plane, a natural hybrid of P. orientalis (oriental plane) and P. occidentalis (American sycamore).[70][71] The parent's names may be kept in their entirety, as seen in Prunus persica × Prunus americana, with the female parent's name given first, or if not known, the parent's names given alphabetically.[72]
Plant species that are genetically compatible may not hybridize in nature for various reasons, including geographical isolation, differences in flowering period, or differences in pollinators. Species that are brought together by humans in gardens may hybridize naturally, or hybridization can be facilitated by human efforts, such as altered flowering period or artificial pollination. Hybrids are sometimes created by humans to produce improved plants that have some of the characteristics of each of the parent species. Much work is now being done with hybrids between crops and their wild relatives to improve disease-resistance or climate resilience for both agricultural and horticultural crops.[73]
Some crop plants are hybrids from different genera (intergeneric hybrids), such as Triticale, × Triticosecale, a wheat–rye hybrid.[74] Most modern and ancient wheat breeds are themselves hybrids; bread wheat, Triticum aestivum, is a hexaploid hybrid of three wild grasses.[30] Several commercial fruits including loganberry (Rubus × loganobaccus)[75] and grapefruit (Citrus × paradisi)[76] are hybrids, as are garden herbs such as peppermint (Mentha × piperita),[77] and trees such as the London plane (Platanus × acerifolia).[78][79] Among many natural plant hybrids is Iris albicans, a sterile hybrid that spreads by rhizome division,[80] and Oenothera lamarckiana, a flower that was the subject of important experiments by Hugo de Vries that produced an understanding of polyploidy.[13]
A sterile hybrid between Trillium cernuum and T. grandiflorum[citation needed]
An ornamental lily hybrid known as Lilium 'Citronella'[81]
Sterility in a non-polyploid hybrid is often a result of chromosome number; if parents are of differing chromosome pair number, the offspring will have an odd number of chromosomes, which leaves them unable to produce chromosomally-balanced gametes.[82] While that is undesirable in a crop such as wheat, for which growing a crop that produces no seeds would be pointless, it is an attractive attribute in some fruits. Triploid bananas and watermelons are intentionally bred because they produce no seeds and are also parthenocarpic.[83]
In humans
There is evidence of hybridisation between modern humans and other species of the genus Homo. In 2010, the Neanderthal genome project showed that 1–4% of DNA from all people living today, apart from most Sub-Saharan Africans, is of Neanderthal heritage. Analyzing the genomes of 600 Europeans and East Asians found that combining them covered 20% of the Neanderthal genome that is in the modern human population.[84] Ancient human populations lived and interbred with Neanderthals, Denisovans, and at least one other extinct Homo species.[85] Thus, Neanderthal and Denisovan DNA has been incorporated into human DNA by introgression.[86]
In 1998, a complete prehistorical skeleton found in Portugal, the Lapedo child, had features of both anatomically modern humans and Neanderthals.[87] Some ancient human skulls with especially large nasal cavities and unusually shaped braincases represent human-Neanderthal hybrids. A 37,000- to 42,000-year-old human jawbone found in Romania's Oase cave contains traces of Neanderthal ancestry[a] from only four to six generations earlier.[89] All genes from Neanderthals in the current human population are descended from Neanderthal fathers and human mothers.[90] A Neanderthal skull unearthed in Italy in 1957 reveals Neanderthal mitochondrial DNA, which is passed on through only the maternal lineage, but the skull has a chin shape similar to modern humans. It is proposed that it was the offspring of a Neanderthal mother and a human father.[91]
En mitologia
Folk tales and myths sometimes contain mythological hybrids; the Minotaur was the offspring of a human, Pasiphaë, and a white bull.[92] More often, they are composites of the physical attributes of two or more kinds of animals, mythical beasts, and humans, with no suggestion that they are the result of interbreeding, as in the centaur (man/horse), chimera (goat/lion/snake), hippocamp (fish/horse), and sphinx (woman/lion).[93] The Old Testament mentions a first generation of half-human hybrid giants, the Nephilim,[94][95] while the apocryphal Book of Enoch describes the Nephilim as the wicked sons of fallen angels and attractive women.[96]
Ver también
- Canid hybrid
- Chimera (genetics)
- Chloroplast capture (botany)
- Eukaryote hybrid genome
- Felid hybrids
- Genetic admixture
- Genetic erosion
- Grex (horticulture)
- Hybridogenesis
- Hybrot
- Inbreeding
- Breeding back
- Interspecific pregnancy
- Xenotransplantation – transplantation of cells or tissue across species
- Horizontal gene transfer
- List of plant hybrids
- List of genetic hybrids
- Macropod hybrids
- Purebred
- Selective breeding
- Genetic use restriction technology
Notas
- ^ Signs of Neanderthal lineage include a wide jaw and large teeth that get bigger toward the back of the mouth.[88]
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enlaces externos
- Artificial Hybridisation – Artificial Hybridisation in orchids
- Domestic Fowl Hybrids
- Hybridisation in animals Evolution Revolution: Two Species Become One, Study Says (nationalgeographic.com)
- Scientists Create Butterfly Hybrid – Creation of new species through hybridization was thought to be common only in plants, and rare in animals.
- What is a human admixed embryo?