RESUMEN
Major genes govern the fertilization of teosinte ovules by maize pollen. A pollen-pistil compatibility system different from the previously described systems, Ga1-s and Tcb1-s, was identified among maize lines introgressed with chromosome segments from 2 teosinte populations. The pistil barrier is dominant, and pollen competence is determined by genotype of the individual pollen grain. A major gene governing this incompatibility behaves as a strong allele of ga2, a locus identified previously among maize genetic stocks on the basis of transmission ratio distortion. Additionally, pollen simultaneously carrying both ga2 and Ga2 was functional on Ga2 silks, which have the pistil barrier, indicating that Ga2 conditions acceptance of the pollen grain rather than ga2 conditioning rejection of the pollen grain by Ga2 silks. The strong allele (Ga2-s), a weaker one such as reported among maize genetic stocks (Ga2-w), and an allele having only pollen competence (Ga2-m), or some combination of these, was found in all 13 of the teosinte populations sampled. Sympatric and parapatric maize landraces carried Ga2-m or the presumed null allele ga2, but Ga2-s or Ga2-w was not found. The combination of exclusively Ga2-s teosinte with ga2 maize, which could provide strong reproductive isolation, was not characteristic of the 5, paired populations tested.
Asunto(s)
Cruzamiento , Genes de Plantas , Polinización , Zea mays/genética , Zea mays/fisiología , Alelos , Cruzamientos Genéticos , Flores/genética , Flores/fisiología , Variación Genética , Genotipo , Endogamia , Óvulo Vegetal/genética , Óvulo Vegetal/fisiología , Fenotipo , Polen/genética , Polen/fisiología , Reproducción/genética , Reproducción/fisiología , Semillas/genética , Semillas/fisiologíaRESUMEN
We describe a family of 3 dominant r1 haplotype-specific enhancers of aleurone color in Zea mays. Stable alleles of the 3 enhancement of r1 loci (enr1, enr2, and enr3) intensify aleurone color conferred by certain pale and near-colorless r1 haplotypes. In addition, unstable alleles of enr1 act on the same set of r1 haplotypes, producing spotted kernels. Components of this instability cross react with the Fcu system of instability. Two of the enr loci are linked with one another but none of the 3 are linked with r1. The r1 haplotypes affected by enr alleles overlap those affected by the inr family of r1 haplotype-specific inhibitors of aleurone color, suggesting a possible interaction.
Asunto(s)
Color , Genes de Plantas , Haplotipos , Pigmentación/genética , Zea mays/genética , Mapeo Cromosómico , Cromosomas de las Plantas , Cruzamientos Genéticos , Elementos Transponibles de ADN/fisiología , Epistasis Genética , Ligamiento Genético , Modelos Biológicos , Mutagénesis Insercional/genética , Mutagénesis Insercional/fisiología , Plantas Modificadas Genéticamente , Sitios de Carácter Cuantitativo/genéticaRESUMEN
A central problem in speciation is the origin and mechanisms of reproductive barriers that block gene flow between sympatric populations. Wind-pollinated plant species that flower in synchrony with one another rely on post-pollination interactions to maintain reproductive isolation. In some locations in Mexico, sympatric populations of domesticated maize and annual teosinte grow in intimate associate and flower synchronously, but rarely produce hybrids. This trait is typically conferred by a single haplotype, Teosinte crossing barrier1-s. Here, we show that the Teosinte crossing barrier1-s haplotype contains a pistil-expressed, potential speciation gene, encoding a pectin methylesterase homolog. The modification of the pollen tube cell wall by the pistil, then, is likely a key mechanism for pollen rejection in Zea and may represent a general mechanism for reproductive isolation in grasses.
Asunto(s)
Hidrolasas de Éster Carboxílico/genética , Fitomejoramiento , Proteínas de Plantas/genética , Aislamiento Reproductivo , Zea mays/genética , Hidrolasas de Éster Carboxílico/metabolismo , Cruzamientos Genéticos , Especiación Genética , Mutación , Proteínas de Plantas/metabolismo , Plantas Modificadas Genéticamente , Tubo Polínico/genética , Tubo Polínico/metabolismo , Simpatría/genéticaRESUMEN
Some populations of maize's closest relatives, the annual teosintes of Mexico, are unreceptive to maize pollen. When present in the pistil (silk and ovary) a number of maize genes discriminate against or exclude pollen not carrying the same allele. An analogous gene Tcb1-s was found in some teosinte populations but not in sympatric or parapatric maize. It was polymorphic among populations of teosinte growing wild, but regularly present in populations growing in intimate association with maize as a weed. Introduction of Tcb1-s into maize substantially to fully restored compatibility with Tcb1-s carrying teosintes. Although Tcb1-s pollen can fertilize tcb1 tcb1 maize, it is at a competitive disadvantage relative to tcb1 pollen. Hence, the influence of Tcb1-s on crossability is bidirectional. In the absence of maize, Tcb1-s can increase in teosinte populations without improving their fitness. In the presence of maize, Tcb1-s appears to have been co-opted to provide reproductive isolation for adaptation to a cultivated habitat.
Asunto(s)
Variación Genética , Genética de Población , Polen/citología , Secuencias Repetitivas de Ácidos Nucleicos/genética , Reproducción/fisiología , Zea mays/genética , Cruzamiento , Cruzamientos Genéticos , Hibridación Genética , Proteínas de Plantas/genética , Semillas/fisiologíaRESUMEN
Paramutation generates heritable changes affecting regulation of specific alleles found at several Zea mays (maize) loci that encode transcriptional regulators of anthocyanin biosynthetic genes. Although the direction and extent of paramutation is influenced by poorly understood allelic interactions occurring in diploid sporophytes, two required to maintain repression loci (rmr1 and rmr2), as well as mediator of paramutation1 (mop1), affect this process at the purple plant1 (pl1) locus. Here we show that the rmr6 locus is required for faithful transmission of weakly expressed paramutant states previously established at both pl1 and red1 (r1) loci. Transcriptional repression occurring at both pl1 and booster1 (b1) loci as a result of paramutation also requires Rmr6 action. Reversions to highly expressed, nonparamutant states at both r1 and pl1 occur in plants homozygous for rmr6 mutations. Pedigree analysis of reverted pl1 alleles reveals variable latent susceptibilities to spontaneous paramutation in future generations, suggesting a quantitative nature of Rmr6-based alterations. Genetic tests demonstrate that Rmr6 encodes a common component required for establishing paramutations at diverse maize loci. Our analyses at pl1 and r1 suggest that this establishment requires Rmr6-dependent somatic maintenance of meiotically heritable epigenetic marks.
Asunto(s)
Epigénesis Genética/genética , Regulación de la Expresión Génica de las Plantas/genética , Genes de Plantas/genética , Patrón de Herencia/genética , Meiosis/genética , Mutación/genética , Zea mays/genética , Antocianinas/biosíntesis , Antocianinas/genética , Cruzamientos Genéticos , Cartilla de ADN , Prueba de Complementación Genética , Genotipo , Repeticiones de Microsatélite/genética , Linaje , Pigmentación/genética , Polen/citologíaRESUMEN
Three genetic systems conferring cross-incompatibility have been described in Zea mays: Teosinte crossing barrier1-strong (Tcb1-s) found in teosinte, and Gametophyte factor1-strong (Ga1-s) and Ga2-s found in maize and teosinte. The reproductive barrier between maize and some weedy teosintes is controlled by the Tcb1-s locus. Multi-generation inheritance experiments on two independent Tcb1-s lineages show that the Tcb1-s barrier is unstable in some maize lines. Reciprocal crosses between Tcb1-s tester plants and three recombinants in the Tcb1-s mapping region demonstrate that the Tcb1-s haplotype contains separable male and female components. In vivo assays of the dynamics of pollen tube growth and pollen tube morphology during rejection of incompatible pollen in silks carrying the Tcb1-s, Ga1-s, or Ga2-s barriers showed that, in all three, pollen tube growth is slower than in compatible crosses at early stages and had ceased by 24 h after pollination. In all three crossing barrier systems, incompatible pollen tubes have clustered callose plugs in contrast to pollen tubes of compatible crosses. Incompatible pollen tubes growing in the Tcb1-s, Ga1-s, and Ga2-s silks have different morphologies: straight, curved, and kinked, respectively. The distinct morphologies suggest that these crossing barriers block incompatible pollen through different mechanisms. This study lays the foundation for cloning the Tcb1 genes and provides clues about the cellular mechanisms involved in pollen rejection in the Tcb1-s, Ga1-s, and Ga2-s crossing barriers.