Abstract
Corolla length is a labile flower feature and has strong implications for pollinator success. However, the phenotypic and genetic bases of corolla elongation are not well known, largely due to a lack of good candidate genes for potential genetic exploration and functional work. We investigate both the cellular phenotypic differences in corolla length, as well as the genetic control of this trait, in Saltugilia (Polemoniaceae). Taxa in this clade exhibit a large range of flower sizes and differ dramatically in pollinator guilds. Flowers of each species were collected from multiple individuals during four stages of flower development to ascertain if cell number or cell size is more important in determining flower size. In Saltugilia, increased flower size during development appears to be driven more by cell size than cell number. Differences in flower size between species are governed by both cell size and cell number, with the large-flowered S. splendens subsp. grantii having nearly twice as many cells as the small-flowered species. Fully mature flowers of all taxa contain jigsaw cells similar to cells seen in sepals and leaves; however, these cells are not typically found in the developing flowers of most species. The proportion of this cell type in mature flowers appears to have substantial implications, comprising 17–68% of the overall flower size. To identify candidate genes responsible for differences in cell area and cell type, transcriptomes were generated for two individuals of the species with the smallest (S. australis) and largest (S. splendens subsp. grantii) flowers across the same four developmental stages visualized with confocal microscopy. Analyses identified genes associated with cell wall formation that are up-regulated in the mature flower stage compared to mid-stage flowers (75% of mature size). This developmental change is associated with the origin of jigsaw cells in the corolla tube of mature flowers. Further comparisons between mature flowers in the two species revealed 354 transcripts that are up-regulated in the large-flowered S. splendens subsp. grantii compared to the small-flowered S. australis. These results are likely broadly applicable to Polemoniaceae, a clade of nearly 400 species, with extensive variation in floral form and shape.
Highlights
The vast morphological diversity observed in angiosperms is often considered to result in part from the interaction between flowers and their pollinators (e.g., Crepet, 1995; Waser et al, 1996; Crepet and Niklas, 2009; Van der Niet and Johnson, 2012; Kearns et al, 2015)
Four taxa of Saltugilia and one of Gilia were grown in the greenhouses at the University of Florida from seeds obtained from Rancho Santa Ana Botanic Garden (Claremont, CA, USA), the Ornamental Germplasm Center at The Ohio State University (Columbus, OH, USA) and Leigh Johnson (Brigham Young University; Provo, UT, USA): G. brecciarum subsp. brecciarum (W6 30785), S. australis (Johnson BYU), S. caruifolia (RSABG 19148), S. splendens subsp. grantii (RSABG 21757) and S. splendens subsp. splendens (RSABG 22676) (Table 1)
For the data set of the complete plastid genome, coverage of the reference plastome ranged from 62.9% (96,715 of 153,853 bp) for S. caruifolia to 99.8% for the field accession of S. splendens subsp. splendens (153,504 of 153,853 bp)
Summary
The vast morphological diversity observed in angiosperms is often considered to result in part from the interaction between flowers and their pollinators (e.g., Crepet, 1995; Waser et al, 1996; Crepet and Niklas, 2009; Van der Niet and Johnson, 2012; Kearns et al, 2015). Changes in floral traits associated with different pollination syndromes, especially traits involving size and number of organs, have been observed to be highly labile (Stebbins, 1974; Givnish, 2002; Lock et al, 2011; Alapetite et al, 2014) and are mostly attributed to shifts in timing, rates and/or patterns of gene expression (Pina et al, 2014). The relative contribution of cell division and cell elongation to corolla growth is debated and likely varies among species (Martin and Gerats, 1993; Stuurman et al, 2004), with the shape of petal epidermal cells having been investigated in many angiosperm families (Kay et al, 1981; Ojeda et al, 2009, 2012). The genetic components of flower size, the control of cell number, cell size and cell shape, are not well understood (Glover, 2007; Ojeda et al, 2012)
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