Evolutionary stasis in Euphorbiaceae pollen: selection and constraints

Abstract

Although much attention has been paid to the role of stabilizing selection, empirical analyses testing the role of developmental constraints in evolutionary stasis remain rare, particularly for plants. This topic is studied here with a focus on the evolution of a pollen ontogenetic feature, the last points of callose deposition (LPCD) pattern, involved in the determination of an adaptive morphological pollen character (aperture pattern). The LPCD pattern exhibits a low level of evolution in eudicots, as compared to the evolution observed in monocots. Stasis in this pattern might be explained by developmental constraints expressed during male meiosis (microsporogenesis) or by selective pressures expressed through the adaptive role of the aperture pattern. Here, we demonstrate that the LPCD pattern is conserved in Euphorbiaceae s.s. and that this conservatism is primarily due to selective pressures. A phylogenetic association was found between the putative removal of selective pressures on pollen morphology after the origin of inaperturate pollen, and the appearance of variation in microsporogenesis and in the resulting LPCD pattern, suggesting that stasis was due to these selective pressures. However, even in a neutral context, variation in microsporogenesis was biased. This should therefore favour the appearance of some developmental and morphological phenotypes rather than others.

Molecular Phylogeny and Pollen Evolution of Euphorbiaceae Tribe Plukenetieae

“…This suggests that the subtribe may be a useful system to explore the underlying mechanisms of aperture reduction and selective pressures for inaperturate pollen, which is a rare and poorly understood condition of eudicots (Furness 2007;Matamoro-Vidal et al 2012). Character optimization on a better resolved phylogeny would help elucidate putative ancestral states for the pollen of Tragiinae, and help quantify the number of transitions to weakly defined aperture and inaperturate conditions.…”

An Australian origin for the candle nut (Aleurites, Crotonoideae, Euphorbiaceae) and the fossil record of the Euphorbiaceae and related families in Australia and New Zealand

“…Combined analysis of rbcL and trnL-F sequence data by Wurdack et al (2005) and Matamoro-Vidal et al (2012) Queensland is outside the modern range of the genus (Fig. 1).…”

“…Martin (1974) recognised that a number of pollen types in the Australian and New Zealand Cenozoic sediments have affinities with the Euphorbiaceae and related families. In this paper we have attempted to constrain, in a phylogenetic sense, the pollen record in Australia and New Zealand within the currently recognised subfamilies in the Euphorbiaceae, and previously included families such as Phyllanthaceae and Picrodendraceae (Wurdack et al, 2004(Wurdack et al, , 2005Matamoro-Vidal et al, 2012, Angiosperm Phylogeny Group IV, 2016.…”

“…Aleurites, and a large group of genera within the Crotonoideae, have inaperturate, spheroidal pollen with an exine sculpture which consists of triangular-rounded structures in A C C E P T E D M A N U S C R I P T 13 regular arrays, known as "croton (or crotonoid) pattern" (Webster, 2014); and this pollen type is a synapomorphy for this "inaperturate" clade (Nowicke, 1994;Wurdack et al, 2005;Tokuoka, 2007;Matamoro-Vidal et al, 2012). Similar inaperturate pollen are recorded from early Oligocene of Western Australia (Macphail and Stone, 2004), from Late Miocene-Early…”

“…Aleurites as a weakly supported sister to a subclade with austral genera Beyeria, Baloghia, Ricinocarpos and Fontainea (Wurdack et al 2005;Matamoro-Vidal et al, 2012) which makes referral of the North American fossil leaves to Aleurites unlikely, and also suggests, a southern origin for the genus. In a recent description of a new crotonoid genus from Africa, this molecular support linking Aleurites to the other Australian genera collapses (Cheek et al, 2016).…”