Pollen[mdash]Stigma Signaling in the Sporophytic Self-Incompatibility Response.

Abstract

INTRODUCTIONMany flowering plants reproduce sexually and generate variation by the process of recombination. Because the possibility of continued evolution is dependent, at least in part, on the production of new variant combinations of genes derived from different individuals, the potential for adaptation is believed to be better under cross-pollination than under self-pollination. In fact, if self-incompatible plants are manipulated to undergo forced self-pollination, they invariably produce progeny with marked inbreeding depression. It is therefore not surprising to find that plants have evolved a variety of mechanisms that favor outcrossing. For example, the crucifer family (Brassicaceae) has an elaborate genetic self-incompatibility (SI) system that controls mating in natural populations of nearly half of the species belonging to this family. This system acts early in pollination to arrest pollen derived from "self." Such plants require cross-pollination to ensure maximal seed production. Radishes, kales, and cabbages are examples of crucifers that rely on outcrossing via cross-pollination by insect vectors to complete their life cycle in the wild. Genetic SI in crucifers is controlled by a complex and highly polymorphic S locus with many specificities or mating reactions. In this review, we discuss the role of the gene products encoded at the S locus in light of recent molecular genetic data derived from the analysis of three Brassica species, B. oleracea, B. campestris, and B. napus. GENETIC AND CYTOLOGICAL FEATURES OF THE SPOROPHYTIC SELF-INCOMPATIBILITY SYSTEM OF BRASSICASI is based on the ability of the pistil to discern the presence of self-pollen and to inhibit the germination or subsequent development of self-related, but not genetically unrelated, pollen. In the Brassicaceae, the genetic control of SI was deciphered in the early 1950s with Bateman's analysis of lberis amara (Bateman, 1955). His genetic model, which has since been generalized to other Brassicaceae and is widely accepted (for review, see Nasrallah and Nasrallah, 1989) includes three basic features: (1) SI is genetically controlled by a single, multiallelicTo whom correspondence should be addressed. locus, the S locus; (2) pollen phenotype is determined not gametophytically by the haploid genotype of the pollen grain but sporophytically by the diploid genotype of the parent plant; and (3) codominant and/or dominant allelic interactions occur that determine the ultimate phenotype of stigma and pollen. The essence of this model is that an incompatible response occurs when the same S allele is active in stigma and pollen. The number of alleles that occur at the S locus is usually large, estimated at 22 in lberis (Bateman, 1955), 34 in Raphanus (Sampson, 1957), and 60 in B. oleracea (Ockendon, 1974).Sporophytically controlled SI systems have been described in members of the Compositae and Convolvulaceae as well as in the Brassicaceae. A common feature of plants with sporophytic SI is that the inhibition of self-pollen is very rapid and oc...

Increased heterosis in selfing populations of a perennial forb

“…We then performed self-pollinations at anthesis by applying pollen (from other flowers on the same plant) with fishing line until the stigma was visibly covered. In sporophytic self-incompatibility, pollen with SI alleles matching those of the maternal plant are prevented from growing down the style ( Nasrallah and Nasrallah 1993 ); thus, we estimated the proportion SI for each line as one minus the proportion of flowers that set fruits (i.e. produced at least one seed; mean seed number per fruit was ∼18).…”

“…Controlled pollinations for estimating inbreeding depression were performed in a similar fashion, with the exception that all pollinations were done at the bud stage ( Kearns and Inouye 1993 ). Because the expression of self-incompatibility factors begins just shortly before anthesis ( Nasrallah and Nasrallah 1993 ), doing crosses at the bud stage allowed us to bypass the SI mechanism. Because of limited numbers of available flowers, these pollinations were done after those for estimating heterosis, and were performed on a unique set of three lines per population.…”

Gene expression and genetic analysis reveal diverse causes of recessive self-compatibility in Brassica napus L.

“…The S locus consists of at least two genes: the stigma determinant S -locus receptor kinase gene ( SRK ) [ 2 ] and the pollen determinant S -locus protein 11 gene ( SP11 )/ S -locus cysteine-rich protein ( SCR ) gene [ 3 , 4 ] ( SP11 , referred to as SCR hereafter). The S locus is also termed the ‘ S haplotype’ because S- locus genes are transmitted to progeny as one unit [ 5 ]. The SRK - SCR interaction is haplotype-specific and only occurs between the receptor and ligand encoded in the same S -locus haplotype [ 6 – 8 ].…”

Commonalities and differences between Brassica and Arabidopsis self-incompatibility

“… 1–3 Because these two genes are tightly linked with each other and inherited as a single Mendelian locus, a set of alleles of the S -locus genes is referred to as S haplotype. 4 The SRK gene is expressed in the stigma papillar cells and encodes a plasma membrane-localized receptor kinase, which has a highly polymorphic extracellular receptor domain (S domain, hereafter) followed by a transmembrane domain and a serine/threonine kinase domain. 1 , 5 Some variants of SRK exhibit more than 30% amino-acid sequence divergence in the S domain.…”