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Abstract:
In flowering plants, intraspecific mate preference is
frequently related to mating systems: the rejection
of self pollen in self-incompatible (SI) plants that
prevents inbreeding is one of the best described examples.
However, in other mating systems, more
nuanced patterns of pollen rejection occur. In the
self-compatible (SC) Nicotiana attenuata, in which
SI is not found and all crosses are compatible,
certain pollen genotypes are consistently selected
in mixed pollinations. However, the molecular
mechanisms of this polyandrous mate selection
remain unknown. Style-expressed NaS-like-RNases
and pollen-expressed NaSLF-like genes, homologous
to SI factors in Solanaceae, were identified
and examined for a role in N. attenuata’s mate selection.
A comparison of two NaS-like-RNases and six
NaSLF-like genes among 26 natural accessions revealed
specific combinations of co-expression and
direct protein-protein interactions. To evaluate their
role inmate selection, we silenced the expression of
specific NaS-like-RNases and NaSLF-like proteins
and conducted diagnostic binary mixed pollinations
and mixed pollinations with 14 different non-self
pollen donors. Styles expressing particular combinations
of NaS-like-RNases selected mates from
plants with corresponding NaS-like-RNase expression
patterns, while styles lacking NaS-like-RNase
expression were non-selective in their fertilizations,
which reflected the genotype ratios of pollen mixtures
deposited on the stigmas. DNA methylation
could account for some of the observed variation
in stylar NaS-like-RNase patterns. We conclude
that the S-RNase-SLF recognition mechanism plays
a central role in polyandrous mate selection in
this self-compatible species. These results suggest
that after the SI-SC transition, natural variation of SI
homologous genes was repurposed to mediate intraspecific mate selection.
