Constraints and selection: insights from microsporogenesis in Asparagales

Abstract

Developmental constraints have been proposed to interfere with natural selection in limiting the available set of potential adaptations. Whereas this concept has long been debated on theoretical grounds, it has been investigated empirically only in a few studies. In this article, we evaluate the importance of developmental constraints during microsporogenesis (male meiosis in plants), with an emphasis on phylogenetic patterns in Asparagales. Different developmental constraints were tested by character reshuffling or by simulated distributions. Among the different characteristics of microsporogenesis, only cell wall formation appeared as constrained. We show that constraints may also result from biases in the correlated occurrence of developmental steps (e.g., lack of successive cytokinesis when wall formation is centripetal). We document such biases and their potential outcomes, notably the establishment of intermediate stages, which allow development to bypass such constraints. These insights are discussed with regard to potential selection on pollen morphology.

Premeiotic Microsporocyte Cell Shape Influences Shape of Tetrads during Microsporogenesis

“…Given that these species belong to different families (Duvall et al 1993), the increase in PMC shape diversity has arisen independently twice. Variation in PMC shape may have evolved because of selection, although this hypothesis remains unclear with regard to pollen morphology since all of the study species produce monosulcate pollen grains (Penet et al 2007). PMC shape variation may also arise as a by-product of other processes, such as polyploidy or genome size (Ressayre et al 2002).…”

“…Second, cell wall formation can be achieved centripetally or centrifugally or even both ways (Penet et al 2005), and it plays a key role during pollen development (Albert et al 2010). Third, postmeiotic sister cells are grouped into tetrads with a dramatic diversity of shapes (Penet et al 2007) that may stay cohesive until anther dehiscence and pollen dispersal (Copenhaver 2005). Fourth, aperture number and exine ornamentation of pollen grains vary dramatically within and among plant families or even at lower taxonomic levels (Nadot et al 2008).…”

“…PENET-PREMEIOTIC MICROSPOROCYTE CELL SHAPE INFLUENCES TETRAD SHAPE sociated with centrifugal cell division that forces pollen into monosulcate shape (Penet et al 2007). Tetrad diversity therefore did not translate into pollen morphological differences, even in species producing frequent irregular tetrads (table 1).…”

Microsporogenesis is simultaneous in the early‐divergent grass Streptochaeta, but successive in the closest grass relative, Ecdeiocolea

“…Anther wall development type from: Bromeliaceae (Sajo et al, 2005), Typha (Asplund, 1972), Sparganium (Mü ller-Doblies, 1969), Rapateaceae (Tiemann, 1985;Venturelli & Bouman, 1988), Prionium (Munro & Linder, 1997), Cyperaceae (Khanna, 1963;Makde, 1982), Juncaceae (Munro & Linder, 1997), Mayaca (Venturelli & Bouman, 1986), Xyris (Rudall & Sajo, 1999), Eriocaulaceae (Monteiro-Scanavacca & Mazzoni, 1978;Arekal & Ramaswamy, 1980), Abolboda (Tiemann, 1985), Restionaceae (Krupko, 1962), Centrolepidaceae (Hamann, 1975), Flagellaria (Munro & Linder, 1997), Anomochlooideae (this paper), Pharoideae (Sajo et al, 2007), other Poaceae (Bhanwra, 1988;Bhanwra et al, 1991 dyads and tetrads are present together, which is unusual. Among other monocots, such reversals in microsporogenesis type can occur occasionally, for example, most lower asparagoids (Asparagales) are simultaneous, with a reversal to the successive type in Hypoxidaceae and within Iridaceae (Rudall et al, 1997;Penet et al, 2007). The presence of simultaneous microsporogenesis in Streptochaeta does not affect pollen morphology.…”

Flowering Ecology of the Species of the Genus Hypoxidia (Hypoxidaceae: Asparagales)

“…Similarly, the level of genetic diversity and gene flow between populations remains unstudied. Research on the genus has been largely restricted to taxonomy [5], phylogenetics [1], and morphology [6][7][8]. Ecological reports are limited to observations of their habitats [5,9] and observation of synchronous flowering [10].…”