16.3E: Autoincompatibilidad - Cómo las plantas evitan la endogamia

La evolución parece favorecer (y estar favorecida por) la variabilidad genética. La variabilidad genética es promovida por la reproducción exogamia - sexual entre padres genéticamente diferentes. Apenas por qué la reproducción sexual es tan popular en todo el mundo de los seres vivos sigue siendo una cuestión muy debatida, pero el hecho permanece. Las plantas, al estar ancladas en posición, tienen un problema especial al respecto. Muchos emplean los servicios de animales (por ejemplo, insectos, aves, murciélagos) para transferir polen de planta a planta. Pero si las flores tienen ambos órganos sexuales: ¿qué es para evitar que el polen fertilice sus propios huevos?

Se han probado una variedad de soluciones en el reino vegetal. Estos incluyen:

• Tener flores imperfectas; es decir, flores que sean masculinas o femeninas.
  • Dioecía. Las flores imperfectas están presentes en plantas separadas. La dioecía es el equivalente de los sexos separados de la mayoría de los animales. Pero es bastante raro. Algunos ejemplos incluyen álamos y hollies.
  • Monoeco. Las flores imperfectas están presentes en la misma planta. Pero si maduran en diferentes momentos, se evita la autofecundación. El maíz (maíz) es un ejemplo común.

Pero la gran mayoría de las angiospermas tienen flores perfectas; es decir, que contiene órganos sexuales tanto masculinos como femeninos. Entonces, ¿cómo evitan la autofecundación?

• Flores heteromórficas.

  Las flores son perfectas pero vienen en dos tipos estructurales; por ejemplo

  • estambres largos con un estilo corto
  • estambres cortos con un estilo largo

• Flores homomórficas. Todas las flores tienen exactamente la misma estructura. La evitación de la autofecundación depende de los mecanismos genéticos/bioquímicos. Hay dos tipos bastante diferentes de autoincompatibilidad.
  • Autoincompatibilidad esporofítica (SSI)
  • Autoincompatibilidad gametofítica (GSI)

Autoincompatibilidad esporofítica (SSI)

Esta forma de autoincompatibilidad ha sido estudiada intensamente en miembros de la familia de la mostaza (Brassica), incluyendo nabos, colza, repollo, brócoli y coliflor.

Figure 16.3.6.1 S Loci

In this system,

• Rejection of self pollen is controlled by the diploid genotype of the sporophyte generation.
• The control lies in the "S-locus", which is actually a cluster of three tightly-linked loci:
  • SLG (S-Locus Glycoprotein) which encodes part of a receptor present in the cell wall of the stigma;
  • SRK (S-Receptor Kinase), which encodes the other part of the receptor. Kinases attach phosphate groups to other proteins. SRK is transmembrane protein embedded in the plasma membrane of the stigma cell.
  • SCR (S-locus Cysteine-Rich protein), which encodes a soluble ligand for the same receptor which is secreted by the pollen.
• Because the plants cannot fertilize themselves, they tend to be heterozygous; that is, carry a pair of different S loci (here designated S₁ and S₂).
• However, dozens of different S alleles may be present in the population of the species; that is; the S-locus in the species is extremely polymorphic (analogous to the major histocompatibility locus of vertebrates).
• The difference between the alleles is concentrated in certain "hypervariable regions" of the receptor (analogous to the hypervariable regions that provide the great binding diversity of antibodies

Figure 16.3.6.3 Pollen Stigma SSI

The rules:

• Pollen will not germinate on the stigma (diploid) of a flower that contains either of the two alleles in the sporophyte parent that produced the pollen.
• This holds true even though each pollen grain being haploid contains only one of the alleles.
• In the example shown here, the S₂ pollen, which was produced by a S₁S₂ parent, cannot germinate on an S₁S₃ stigma.

The explanation:

• The S₁S₂ pollen-producing sporophyte synthesizes both SCR₁ and SCR₂ for incorporation in (and later release from) both S₁ and S₂ pollen grains.
• If either SCR molecule can bind to either receptor on the pistil, the kinase triggers a series of events that lead to failure of the stigma to support germination of the pollen grain. Among these events is the ubiquination of proteins targeting them for destruction in proteasomes.
• If this path is not triggered (e.g., pollen from an S₁S₂ parent on an S₃S₄ stigma, the pollen germinates successfully.

Gametophytic Self-Incompatibility (GSI)

Figure 16.3.6.4 GSI

This form of self-incompatibility is more common than SSI but not so well understood. It occurs in nearly one-half of all the families of angiosperms, including

• the Solanaceae (potatoes, tomatoes [wild, not cultivated], and tobacco)
• petunias
• beets (Beta vulgaris)
• buttercups (Ranunculus)
• lilies
• roses
• many grasses

The rules:

• The S loci are (as in SSI plants) extremely polymorphic; that is, there is an abundance of multiple alleles in the population.
• Incompatibility is controlled by the single S allele in the haploid pollen grain.
• Thus a pollen grain will grow in any pistil that does not contain the same allele (so, as shown here and in contrast to what happens in SSI, S₂ pollen from an S₁S₂ parent will grow down an S₁S₃ style.

This appears to be the mechanism in the petunia:

• All pollen grains - incompatible as well as compatible - germinate forming pollen tubes that begin to grow down the style.
• However, growth of incompatible pollen tubes stops in the style while compatible tubes go on to fertilize the egg in the ovary.
• The block within incompatible pollen tubes is created by an S-locus-encoded ribonuclease (S-RNase), which is synthesized within the style, enters the pollen tube and destroys its RNA molecules halting pollen tube growth.
• The RNase molecules contain a hypervariable region, each encoded by a different allele, which establishes each S specificity (S₁, S₂, S₃, etc.).
• The pollen tube expresses a protein designated SLF that binds S-RNase. SLF also exists in different S specificities (S₁, S₂, S₃, etc.).
• In compatible ("nonself") tubes, the SCF triggers the degradation (in proteasomes) of the S-RNase thus permitting RNAs in the pollen tube to survive and growth to continue.
• In incompatible ("self") tubes the interaction of, for example, the S₁ SCF with the S₁ S-RNase blocks its degradation so the RNAs of the pollen tube are destroyed and growth is halted.

An entirely-different mechanism of gametophytic self-incompatibility is found in poppies (Papaver rhoeas).

Switching from Cross-Pollination to Self-Pollination

A substantial minority of angiosperms have abandoned cross-pollination for self-pollination. For example, while its wild relatives continue to be cross-pollinated, the domestic tomato is not. Two steps are needed for this change:

1. abandoning its mechanism of self-incompatibility
2. changes in flower structure to reduce the chance that pollinators will transfer pollen from another plant to its stigma.

Unlike its wild relatives, the stigma of the domestic tomato does not protrude beyond the anthers. Of the several genes involved in this change, the most important one is Style2.1. The mutation in Style2.1 responsible for the change in phenotype in our cultivated tomatoes is found in the promoter region - the protein-encoding portion of the gene is exactly the same as in wild tomatoes.

Here, again, is evidence that much of the diversity of life arises not from mutations in the protein-coding portion of the genes that we share but mutations in their regulatory regions (promoters and enhancers).

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