Recent speciation ofCapsella rubellafromCapsella grandiflora, associated with loss of self-incompatibility and an extreme bottleneck

Abstract

Flowering plants often prevent selfing through mechanisms of self-incompatibility (S.I.). The loss of S.I. has occurred many times independently, because it provides short-term advantages in situations where pollinators or mates are rare. The genus Capsella, which is closely related to Arabidopsis, contains a pair of closely related diploid species, the self-incompatible Capsella grandiflora and the self-compatible Capsella rubella. To elucidate the transition to selfing and its relationship to speciation of C. rubella, we have made use of comparative sequence information. Our analyses indicate that C. rubella separated from C. grandiflora recently (Ϸ30,000 -50,000 years ago) and that breakdown of S.I. occurred at approximately the same time. Contrasting the nucleotide diversity patterns of the 2 species, we found that C. rubella has only 1 or 2 alleles at most loci, suggesting that it originated through an extreme population bottleneck. Our data are consistent with diploid speciation by a single, selfing individual, most likely living in Greece. The new species subsequently colonized the Mediterranean by Northern and Southern routes, at a time that also saw the spread of agriculture. The presence of phenotypic diversity within modern C. rubella suggests that this species will be an interesting model to understand divergence and adaptation, starting from very limited standing genetic variation.

The ARC1 E3 Ligase Promotes Two Different Self-Pollen Avoidance Traits inArabidopsis

“…After binding, SRK activates a signaling pathway in the stigmatic papilla that rapidly rejects pollen by blocking pollen hydration or pollen tube penetration into the stigmatic surface (reviewed in Chapman and Goring, 2010;Ivanov et al, 2010). The SCR/SP11 and SRK genes are highly polymorphic, and alleles have been identified in other species, including A. lyrata (Kusaba et al, 2001;Schierup et al, 2001Schierup et al, , 2006Prigoda et al, 2005;Mable and Adam, 2007) and C. grandiflora (Paetsch et al, 2006;Boggs et al, 2009b;Guo et al, 2009). Downstream of SRK in the self-incompatibility pathway, there are two positive regulators that have been shown to interact with SRK, M locus Protein Kinase (MLPK) and the E3 ubiquitin ligase, ARM-Repeat Containing1 (ARC1) (Gu et al, 1998;Kakita et al, 2007aKakita et al, , 2007b.…”

The Capsella rubella genome and the genomic consequences of rapid mating system evolution

“…The highly selfing species C. rubella, found throughout much of southern and western Europe, separated less than 200,000 years ago from the self-incompatible, obligate outcrosser C. grandiflora, which is restricted primarily to the northwest of Greece 8,9 . In contrast to Arabidopsis, the breakdown of self incompatibility in Capsella was concurrent with species divergence [8][9][10] .…”

Signaling Events in Pollen Acceptance or Rejection in the Arabidopsis Species

“…These vesicles are hypothesized to deliver factors to allow for the transfer of water from the papilla to the pollen grain and to deliver cell wall modifi cation enzymes. The compatible pollen grain is then able to hydrate, and the pollen tube penetrates the stigma to continue onto an ovule or fertilization Brassica species (Haasen and Goring 2010 ), and have been characterized in other Brassicaceae species, including A. lyrata (Kusaba et al 2001 ;Schierup et al 2001 ;Mable et al 2005 ) and Capsella grandifl ora (Paetsch et al 2006 ;Boggs et al 2009 ;Guo et al 2009 ). Although much of the research into the self-incompatibility pathway has been performed in the Brassica genus, recent discoveries in the Arabidopsis genus are described here.…”