Flower Development and the Evolution of Self-fertilization in Amsinckia: The Role of Heterochrony

Abstract

We studied the development of 26 flower traits under natural conditions in three clades of the genus Amsinckia (Boraginaceae). Each clade contained both a derived highly self-fertilizing taxon and an ancestral more highly outcrossing taxon. The more outcrossing taxa contained two flower morphs-pins and thrums-with opposite positioning of the sex organs (heterostyly). The highly selfing taxa had smaller flowers with sex organs in close proximity (homostyly). Growth trajectories were quantified over the entire or nearly the entire period from primordium initiation to flower opening. These trajectories were compared in the heterochronic framework and, in contrast with previous studies, character size was tracked over time rather than relative to another character. We focused on three hypotheses: (1) The distinct developmental trajectories leading to pins and thrums should be similar in all clades, while the trajectories leading to homostylous flowers might differ among clades. This was supported. Specifically, contrasting growth rates of stamen and pistil heights in heterostylous flowers caused pin and thrum flowers to have the reciprocal arrangement of anther and stigma heights. From the viewpoint of heterochrony, the decreased size (paedomorphosis) of the homostylous morph, compared to pins and thrums, resulted from decreased growth rate (neoteny) and earlier offset (progenesis) in all clades. Nevertheless, multiple heterochronic processes were involved in the mosaic development and evolution of homostylous flowers. (2) We tested the hypothesis that small, self-fertilizing flowers have reduced development times, one of the proposed selective advantages of increased self-fertilization rates. We found in contrast that developmental duration of homostylous flowers was either the same (two clades) or longer (one clade) compared to duration of pins and thrums. (3) Finally, we tested von Baer's Law, which proposes that developmental differences among closely related taxa should arise later in development than differences among more distantly related taxa. Von Baer's Law was supported strongly among homostyles, moderately among thrums and weakly among pins.

Patterns of Inflorescence Development of Three Prairie Grasses (Andropogoneae, Poaceae)

“…The "law" was translated to plants by Tucker (1997) as "the hierarchicalsignificance hypothesis," in which she postulated that characters of families will appear in early development, whereas characters of genera appear later and characters of species later still. The hypothesis has been supported by Tucker's own work on legumes and by some aspects of development in Amsinckia (Boraginaceae; Li and Johnston 2010).…”

De novo Sequencing and Comparative Transcriptomics of Floral Development of the Distylous Species Lithospermum multiflorum

“…The species is a member of Boraginaceae, the family that has the largest number of independent origins of heterostyly, and this type of breeding system has evolved multiple times in Lithospermum L. (Cohen, 2011). Cohen et al (2012) demonstrated that there are differences in the development of the flowers within the genus, and data concerning SI and floral morphology and development based on studies by Philipp and Schou (1981), Casper (1985), Li and Johnston (2010), Ferrero et al (2012), and Cohen (2014) suggest that heterostyly evolved following distinct patterns in different tribes of the family. Heterostyly arose independently in L. mulitflorum , and the species is sister to Lithospermum macromeria .…”

Complex heterochrony underlies the evolution ofCaenorhabditis eleganshermaphrodite sex allocation

“…). Despite these studies, however, it remains mostly unknown which developmental changes underlie the evolution of hermaphrodite sex allocation under selfing and/or outcrossing (Li and Johnston ; Roels and Kelly ; Noel et al. ).…”