Abstract
BackgroundFlower size varies dramatically across angiosperms, representing innovations over the course of >130 million years of evolution and contributing substantially to relationships with pollinators. However, the genetic underpinning of flower size is not well understood. Saltugilia (Polemoniaceae) provides an excellent non-model system for extending the genetic study of flower size to interspecific differences that coincide with variation in pollinators.ResultsUsing targeted gene capture methods, we infer phylogenetic relationships among all members of Saltugilia to provide a framework for investigating the genetic control of flower size differences via RNA-Seq de novo assembly. Nuclear concatenation and species tree inference methods provide congruent topologies. The inferred evolutionary trajectory of flower size is from small flowers to larger flowers. We identified 4 to 10,368 transcripts that are differentially expressed during flower development, with many unigenes associated with cell wall modification and components of the auxin and gibberellin pathways.ConclusionsSaltugilia is an excellent model for investigating covarying floral and pollinator evolution. Four candidate genes from model systems (BIG BROTHER, BIG PETAL, GASA, and LONGIFOLIA) show differential expression during development of flowers in Saltugilia, and four other genes (FLOWERING-PROMOTING FACTOR 1, PECTINESTERASE, POLYGALACTURONASE, and SUCROSE SYNTHASE) fit into hypothesized organ size pathways. Together, these gene sets provide a strong foundation for future functional studies to determine their roles in specifying interspecific differences in flower size.
Highlights
Flower size varies dramatically across angiosperms, representing innovations over the course of >130 million years of evolution and contributing substantially to relationships with pollinators
The goals of this study are two-fold: first, we investigate phylogenetic relationships within Saltugilia using current analytical methods for next-generation sequencing data, as well as a comparison between concatenation and species tree inference methods; second, we investigate patterns of differential gene expression associated with differences in flower size in representatives of the six taxa of Saltugilia (S. australis, S. caruifolia, S. latimeri, S. splendens subsp. grantii, S. splendens subsp. splendens (GH; denoting that this sample was grown in the greenhouse common garden), and S. splendens subsp. splendens (FS; denoting that this is plant material collected in the field)) using RNA-Seq methods
In Saltugilia, we found a homolog of FLOWERING-PROMOTING FACTOR 1, which has been shown to be involved in gibberellin signaling and may lead to enhanced responsiveness to gibberellins [75], with additional implications for changes in epidermal cell shape and formation of trichomes of leaves in overexpression studies [76]
Summary
Flower size varies dramatically across angiosperms, representing innovations over the course of >130 million years of evolution and contributing substantially to relationships with pollinators. Saltugilia (Polemoniaceae) provides an excellent nonmodel system for extending the genetic study of flower size to interspecific differences that coincide with variation in pollinators. Landis et al BMC Genomics (2017) 18:475 These de novo approaches are ideal for studies involving the phlox family (Polemoniaceae). There are no genome-level resources available for any members of Polemoniaceae, and only two studies have been published using transcriptomic data: one on a single species of Phlox [22] and the other on two species of Saltugilia [23]. Comparative studies focused on closely related species in Polemoniaceae have shown drastic differences in flower size in species of Ipomopsis [24], Leptosiphon [25], and Saltugilia [23]. The hypothesized gene networks of many, but not all, of these genes have been reviewed and outlined [35, 36], but work remains, in non-model species, to clarify if – and how – these genes determine flower size
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