Heritable Variation in a Family-Diagnostic Trait

Abstract

Abstract. Derived characters that have not changed during the diversification of a clade provide traits that are diagnostic at higher taxonomic levels. The tetradynamous stamen condition (four long and two short stamens) of the Brassicaceae is an example of a diagnostic trait that has not changed during the diversification of this large flowering plant family. We investigated one hypothesis that might explain the long-term stasis of this trait-that tetradynamous stamens have persisted because of an absence of genetic variation underlying the trait. Through a sib-analysis with Raphanus raphanistrum and an artificial selection experiment with Brassica rapa, we demonstrate that significant genetic variation is present for the tetradynamous condition in both species and that the trait is therefore not constrained from evolutionary change by a lack of heritable genetic variation.Key words. Artificial selection, Brassicaceae, Brassica rapa, floral evolution, genetic constraints, genetic variation, Raphanus raphanistrum.Received December 23, 1998. Accepted February 9, 2000 Characters that distinguish taxa at the level of family or order are likely to have originated with the clade's common ancestor and remained static during the subsequent divergence of species within the taxonomic group. The processes that govern the evolution of these traits that are diagnostic for higher taxa of flowering plants have received little empirical investigation. Although developmental homeostasis may contribute to the relative invariance of such traits within a species (Fenster and Galloway 1997;Cresswell 1998), evolutionary explanations that might account for the stasis across species demonstrated by diagnostic characters include either stabilizing selection or some form of evolutionary constraint. Potential constraints include an absence of genetic variation underlying the character or genetic correlations with traits that are themselves stabilized by selection (Maynard Smith et al. 1985). We have investigated the heritability of a floral trait that is diagnostic for the family Brassicaceae to determine if an absence of genetic variation might constrain the evolution of this taxonomically informative character. Our results for two species of mustard show that heritable variation is present for the trait, indicating that other hypotheses must be invoked to account for the trait's evolutionary stasis.The majority of the 3000 species in the mustard clade (family Brassicaceae or Cruciferae) possess flowers in which there are four medial stamens that are long and two lateral stamens that are short (the ''tetradynamous'' condition); this character state is considered diagnostic for the family (Cronquist 1981;Endress 1992;Heywood 1993;Zomlefer 1994). The possession of dimorphic stamen lengths cuts across the diversity of mating systems and pollination modes shown by mustard taxa, which range from autogamously selfing to obligately outcrossing, self-incompatible species (Preston 1986). Although the adaptive nature of specific floral traits has...

Temperature-Dependent Fluctuation Of Stamen Number InCardamine hirsuta(Brassicaceae)

“…On the other hand, we still do not know the function of this instability or the consequences in terms of reproductive success of C. hirsuta, nor do we know whether there are ecological consequences of the temperature-controlled variation in stamen number. The function of tetradynamous stamens in Brassicaceae is still an open question (Karoly and Conner 2000). The phenomena observed in this study will provide further opportunities to identify both the mechanistic cause and the ecological consequences of the tetradynamous pattern of stamen arrangement.…”

“…In addition, a wide array of genetic variants may be produced because of mutation, outcrossing, and recombination. It has also been reported that there is heritable variation in the tetradynamous pattern of stamens in Brassicaceae, at least in terms of relative lengths of medial and lateral stamens in Raphanus raphanistrum and Brassica rapa (Karoly and Conner 2000).…”

Genetic Variance–covariance Matrices: A Critique of the Evolutionary Quantitative Genetics Research Program

“…For example, most of the species of the plant family Brassicaceae have flowers characterized by two sets of stamens (the male sexual organs), in which four stamens are longer than the remaining two. While the reason for this is far from being clear (Karoly and Conner 2000;Conner 2002), the size of the two sets of stamens will show covariation at the taxonomically high level of an entire family of plants. On the other hand, it is well known that many traits can be uncoupled not merely within a population or species, but even within an individual organism!…”

Patterns and Mechanisms of Selection on a Family-Diagnostic Trait: Evidence From Experimental Manipulation and Lifetime Fitness Selection Gradients

“…While a rigorous comparative study of tetradynamy has not been done, two genera that lack strong dimorphism (Stanleya and Thelungiella) are derived taxa in recent molecular phylogenies of the Brassicaceae (Price et al 1994;Galloway et al 1998;Barnes 2001). Karoly and Conner (2000) found evidence of genetic variation underlying tetradynamous stamens in two closely related Brassicaceae, Raphanus raphanistrum and Brassica rapa, thus refuting the lack of variation hypothesis for these species. Our study was designed to test one of the other evolutionary explanations for conservation of dimorphic anther heights-stabilizing selection.…”

“…Evolutionary explanations for phenotypic invariability at a high taxonomic level include a lack of genetic variation for the trait, constraints due to genetic correlations or developmental processes, or stabilizing selection (Maynard Smith et al 1985;Karoly and Conner 2000;Gould 2002). These three hypotheses are not mutually exclusive, and they may not be equally suitable for explaining stasis at different taxonomic levels.…”