Bridging the gaps: evolution and development of perianth fusion

Abstract

330I.330II.331III.332IV.333334References334 Summary One of the most striking innovations in flower development is the congenital or postgenital union of petals (sympetaly) which has enabled dramatic specialization in flower structure and possibly accelerated speciation rates. Sympetalous flowers exhibit extraordinary variation in development, including the degree and timing of fusion, and fusion with other floral organs. Different axes of corolla tube complexity can be disentangled at the developmental level, with most variation being explained by differences in coordinated growth between interconnected and lobed regions of neighboring petal primordia, and between lower and upper portions of the corolla tube, defined by the stamen insertion boundary. Genetically, inter‐ and intra‐specific variation in the degree of petal fusion is controlled by various inputs from genes that affect organ boundary and lateral growth, signaling between different cell types, and production of the cuticle. It is thus hypothesized that the evolution and diversification of fused petals, at least within the megadiverse Asteridae clade of core eudicots, have occurred through the modification of a conserved genetic pathway previously involved in free petal development.

The best of both worlds? A review of delayed selfing in flowering plants

“…For example, delayed selfing occurred in a Clematis species (Ranunculaceae) with tubular flowers and a specialized bee pollination syndrome, whereas its outcrossing congener had open flowers and more generalist pollination (Jiang et al, 2010). As broad evidence, we point to a prevalence of fused corollas (30 genera with fused vs. 21 with free petals, consistent with the trend for a high frequency of asterids), which are thought to be associated with pollinator specialization (Wernham, Goodwillie et al, 2004 (continued) 1911; Faegri and Van Der Pijl, 1979;Zhong and Preston, 2015). Furthermore, a single pollinator functional group (e.g., hummingbirds, moths, bees) was reported in 27 of 60 species for which we found any pollinator information.…”

Implications of region‐specific gene expression for development of the partially fused petunia corolla

“…Along with the fact that mutations in boundary genes of many taxa often have no impact on petal fusion (Weir et al ., ; Aida and Tasaka, ,; Takeda et al ., ), our data suggest that the mechanism of organ boundary formation is quite different between petals and other lateral organs. Thus, the expectation that sympetaly results from the regional ‘turning‐off’ of known organ boundary genes (Zhong and Preston, ; Rebocho et al ., ), might be somewhat naïve.…”

“…Along with the fact that mutations in boundary genes of many taxa often have no impact on petal fusion (Weir et al, 2004;Tasaka, 2006a,2006b;Takeda et al, 2011), our data suggest that the mechanism of organ boundary formation is quite different between petals and other lateral organs. Thus, the expectation that sympetaly results from the regional 'turning-off' of known organ boundary genes (Zhong and Preston, 2015;Rebocho et al, 2017), might be somewhat na€ ıve. Petal growth is best characterized in A. thaliana and occurs through the establishment of a distal, auxin-mediated polarity field that increases growth rates towards the distal end of the petal, resulting in the characteristic spoonshaped appearance (Sauret-Gueto et al, 2013).…”

“…Tucker, ), this makes the sympetalous (fused petal) unit capable of developing a variety of shapes to better manipulate the orientation of pollinator entry into the flower (Fenster et al ., ; Krishna and Keasar, ). Candidate genes proposed to be involved in the evolution of sympetaly and/or adnation of stamens to petals include, but are certainly not limited to, LSH3/OB1 ‐, WAX INDUCER1/SHINE1 ( WIN1 / SHN1 )‐, HAN ‐ and CUC ‐like genes (Specht and Howarth, ; Zhong and Preston, ; Zhong et al ., ; Rebocho et al ., ). Overexpression of LSH3 / OBO1 in A. thaliana causes fusion between stamens and petals, providing a potential mechanism for organ fusion through its upregulation at the petal margins (Zhong and Preston, ).…”

“…Candidate genes proposed to be involved in the evolution of sympetaly and/or adnation of stamens to petals include, but are certainly not limited to, LSH3/OB1 ‐, WAX INDUCER1/SHINE1 ( WIN1 / SHN1 )‐, HAN ‐ and CUC ‐like genes (Specht and Howarth, ; Zhong and Preston, ; Zhong et al ., ; Rebocho et al ., ). Overexpression of LSH3 / OBO1 in A. thaliana causes fusion between stamens and petals, providing a potential mechanism for organ fusion through its upregulation at the petal margins (Zhong and Preston, ). Similarly, loss‐of‐function win1 / shn1 mutants show increased organ fusion due to the decreased synthesis of extracellular cuticular waxes (Oshima et al ., ).…”