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We study self-incompatibility (SI), a genetic barrier to self-fertilisation that has arisen repeatedly during the evolution of flowering plants.   Self-fertilisation can occur following pollen transfer within a flower (self-pollination) or between flowers of the same plant (geitonogamous pollination).   Remarkably, the flowers of a self-incompatible plant can distinguish the source of each pollen grain it receives and only let those pollen grains that are from another plant fertilise its precious cargo of ovules.   Because SI prevents fertilisation following self and geitonogamous pollinations, self-incompatible plants are obligate outcrossers.

We study SI in the Solanaceae, a large family of flowering plants that includes such familiar plants as tobacco, tomato and petunia.   SI in this family is of the gametophytic type and is controlled by a single genetic locus called the S locus.   The figure shows how the S locus in a gametophytic SI system works to reject incompatible pollen.

There are a large number of S alleles at the S locus.   Being diploid, each flower has two S alleles.   Pollen grains have just one S allele because they are haploids.   A pollen grain that alights on flower with an S allele that matches its own is rejected.   Only those pollen grains that alight on a flower with two non-matching S alleles can fertilise an ovule.

From this simple biology, it is clear that the S locus must be expressed in both the pollen grain and pistil, which is the name given to the female reproductive tissues of the flower.  To date, molecular studies have shown that the product of the S locus in the pistil is a ribonuclease called the S-RNase.   This enzyme has to be enzymatically active for pollen rejection to occur.   Less is known about the product of the S locus in the pollen grain, except that it appears to be a member of the F-box family of proteins, which are proteins that help select other proteins for degradation through the ubiquitin pathway.   How the S-RNase interacts with the F-box protein, and how this interaction leads to the rejection of an incompatible pollen tube, are some of the mysteries we are exploring using the tools of cellular and molecular biology.   The population genetics and evolution of SI systems are also areas we study.

RECENT PUBLICATIONS

Newbigin E and Vierstra R D (2003)   Plant reproduction: Sex and self-denial.   Nature 423, 229-230.

Takebayashi N, Brewer PB, Newbigin E and Uyenoyama MK (2003)   Patterns of variation within self-incompatibility loci.   Molecular Biology and Evolution 20, 1778-1794.

Uyenoyama MK, Zhang Y and Newbigin, E (2001)   On the origin of self-incompatibility haplotypes: Transition through self-compatible intermediates.   Genetics 157, 1805-1817.

MacIntosh GC, Bariola PA, Newbigin E and Green PJ (2001)   Characterization of Rny1, the Saccharomyces cerevisiae member of the T 2 RNase family of RNases: Unexpected functions for ancient enzymes? Proceedings of the National Academy of Sciences, USA 98,1018-1023.

Golz JF, Oh H-Y, Su V, Kusaba M, Newbigin, E (2001)   Genetic analysis of Nicotiana pollen-part mutants is consistent with the presence of an S-ribonuclease inhibitor at the S locus.   Proceedings of the National Academy of Sciences, USA 98, 15372-15376.

Newbigin, E and Uyenoyama MK (2000)   Evolutionary dynamics of dual-specificity self-incompatibility alleles.   Plant Cell 12, 310-312.

Li J-H, Nass N, Kusaba M, Dodds PN, Treloar N, Clarke AE and Newbigin E (2000) A genetic map of the Nicotiana alata S locus that includes three pollen-expressed genes.   Theoretical and Applied Genetics 100, 956-964.

Golz JF, Clarke AE and Newbigin E (2000)   Mutational approaches to the study of self-incompatibility: Revisiting the pollen-part mutants.   Annals of Botany 85, 95-103.

Golz JF, Su V, Clarke AE and Newbigin E (1999) A molecular description of mutations affecting the pollen component of the Nicotiana alata S locus.   Genetics 152, 1123-1135.