Evolutionary Genetics of an S-Like Polymorphism in Papaveraceae with Putative Function in Self-Incompatibility

Papaver rhoeas possesses a gametophytic self-incompatibility (SI) system not homologous to any other SI mechanism characterized at the molecular level. Four previously published full length stigmatic S-alleles from the genus Papaver exhibited remarkable sequence divergence, but these studies failed to amplify additional S-alleles despite crossing evidence for more than 60 S-alleles in Papaver rhoeas alone.Using RT-PCR we identified 87 unique putative stigmatic S-allele sequences from the Papaveraceae Argemone munita, Papaver mcconnellii, P. nudicuale, Platystemon californicus and Romneya coulteri. Hand pollinations among two full-sib families of both A. munita and P. californicus indicate a strong correlation between the putative S-genotype and observed incompatibility phenotype. However, we also found more than two S-like sequences in some individuals of A. munita and P. californicus, with two products co-segregating in both full-sib families of P. californicus. Pairwise sequence divergence estimates within and among taxa show Papaver stigmatic S-alleles to be the most variable with lower divergence among putative S-alleles from other Papaveraceae. Genealogical analysis indicates little shared ancestral polymorphism among S-like sequences from different genera. Lack of shared ancestral polymorphism could be due to long divergence times among genera studied, reduced levels of balancing selection if some or all S-like sequences do not function in incompatibility, population bottlenecks, or different levels of recombination among taxa. Preliminary estimates of positive selection find many sites under selective constraint with a few undergoing positive selection, suggesting that self-recognition may depend on amino acid substitutions at only a few sites.Because of the strong correlation between genotype and SI phenotype, sequences reported here represent either functional stylar S-alleles, tightly linked paralogs of the S-locus or a combination of both. The considerable complexity revealed in this study shows we have much to learn about the evolutionary dynamics of self-incompatibility systems

Combining S-cone and luminance signals adversely affects discrimination of objects within backgrounds

The visual system processes objects embedded in complex scenes that vary in both luminance and colour. In such scenes, colour contributes to the segmentation of objects from backgrounds, but does it also affect perceptual organisation of object contours which are already defined by luminance signals, or are these processes unaffected by colour’s presence? We investigated if luminance and chromatic signals comparably sustain processing of objects embedded in backgrounds, by varying contrast along the luminance dimension and along the two cone-opponent colour directions. In the first experiment thresholds for object/non-object discrimination of Gaborised shapes were obtained in the presence and absence of background clutter. Contrast of the component Gabors was modulated along single colour/luminance dimensions or co-modulated along multiple dimensions simultaneously. Background clutter elevated discrimination thresholds only for combined S-(L + M) and L + M signals. The second experiment replicated and extended this finding by demonstrating that the effect was dependent on the presence of relatively high S-(L + M) contrast. These results indicate that S-(L + M) signals impair spatial vision when combined with luminance. Since S-(L + M) signals are characterised by relatively large receptive fields, this is likely to be due to an increase in the size of the integration field over which contour-defining information is summed