Faculty of Science, School of Sciences, Natabua Campus Lautoka BIO706 Embryology Lecture 25: Self-incompatibility - I

Breeding systems enforcing outcrossing Evolutionarily advantageous (in theory) to prevent pollination between closely related individuals major mechanisms enforcing outcrossing (cross-pollination)

self-incompatibility—negative chemical interaction between pollen and style tissue with same alleles heterostyly—mechanical prevention of pollen deposition by relative placement of anthers to style dioecy—separation of anthers and pistils on separate plants

Self-incompatibility (SI) It refers to the inability of a plant with functional pollen to set seeds when self pollinated. It is the failure of pollen from a flower to fertilize the same flower or other flowers of the same plant.

Self-incompatibility (SI) is a general name for several genetic mechanisms in angiosperms, which prevent self-fertilization and thus encourage out-crossing and allogamy.

In plants with SI, when a pollen grain produced in a plant reaches a stigma of the same plant or another plant with a similar genotype, the process of pollen germination, pollen tube growth, ovule fertilization, and embryo development is halted at one of its stages, and consequently no seeds are produced.

SI is one of the most important means to prevent selfing and promote the generation of new genotypes in plants, and it is considered as one of the causes for the spread and success of the angiosperms on the earth.

Classification of Self-incompatibility Criteria Types Flower morphology Heteromorphic self incompatibility Distyly Tristyly Homomorphic self incompatibility Sporophytic self incompatibility Gametophytic self incompatibility

Criteria Types Genes involved (number) Monoallelic (governed by single gene) Diallelic (governed by two genes) Polyallelic (governed by many genes) Cytology of pollen Binucleate (pollens with two nuclei) Trinucleate (pollens with three nuclei) Expression site Ovarian (expression site is ovary) Stylar (expression site is style) Stigmatic (expression site is stigma)

General features of Self-incompatibility Prevents selfing and promotes out-breeding so increases the probability of new gene combinations. Causes may be morphological, physiological, genetical or biochemical.

Normal seed set on cross pollination. May operate at any stage between pollination and fertilization. Reduces homozygosity. In plants, self-incompatibility is often inherited by a single gene (S) with different alleles (e.g. S1, S2, S3 etc.) in the species population

Types of self incompatibility Single-locus self-incompatibility Gametophytic self-incompatibility (GSI) Sporophytic self-incompatibility (SSI) 2) 2-loci gametophytic self-incompatibility. 3) Heteromorphic self-incompatibility Cryptic self-incompatibility (CSI) Late-acting self-incompatibility (LSI)

Mechanisms of single-locus self-incompatibility The best studied mechanisms of SI act by inhibiting the germination of pollen on stigmas, or the elongation of the pollen tube in the styles. These mechanisms are based on protein-protein interactions, and the best-understood mechanisms are controlled by a single locus termed S, which has many different alleles in the species population.

Despite their similar morphological and genetic manifestations, these mechanisms have evolved independently, and are based on different cellular components; therefore, each mechanism has its own, unique S-genes.

The S-locus contains two basic protein coding regions - one expressed in the pistil, and the other in the anther and/or pollen (referred to as the female and male determinants, respectively). Because of their physical proximity, these are genetically linked, and are inherited as a unit. The units are called S-haplotypes.

The translation products of the two regions of the S-locus are two proteins which, by interacting with one another, lead to the arrest of pollen germination and/or pollen tube elongation, and thereby generate an SI response, preventing fertilization. However, when a female determinant interacts with a male determinant of a different haplotype, no SI is created, and fertilization ensues.

This is a simplistic description of the general mechanism of SI, which is more complicated, and in some species the S-haplotype contains more than two protein coding regions.

Gametophytic Self-Incompatibility (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 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

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(S-locus F-box protein) that binds S-RNase. SLF also exists in different S specificities (S1, S2, S3, etc.).

In compatible ("nonself") tubes, the SLF or SCF (Skp1–Cul1–F-box-protein ubiquitin ligase) 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.

Cross Compatibility S1S2 X S1S2 Fully incompatible S1S2 X S1S3 Partially compatible S1S2 X S3S4 Fully compatible

Thank You Questions are welcome