Expression of ovule and integument‐associated genes in reduced ovules of Santalales

Abstract

Santalales comprise mainly parasitic plants including mistletoes and sandalwoods. Bitegmic ovules similar to those found in most other angiosperms are seen in many members of the order, but other members exhibit evolutionary reductions to the unitegmic and ategmic conditions. In some mistletoes, extreme reduction has resulted in the absence of emergent ovules such that embryo sacs appear to remain embedded in placental tissues. Three santalalean representatives (Comandra, Santalum, and Phoradendron), displaying unitegmic, and ategmic ovules, were studied. Observed ovule morphologies were consistent with published reports, including Phoradendron serotinum, which we interpret as having reduced ategmic ovules, consistent with earlier reports on this species. For further understanding of the nature of the ovule reductions we isolated orthologs of the Arabidopsis genes AINTEGUMENTA (ANT) and BELL1 (BEL1), which are associated with ovule development in this species. We observed ovular expression of ANT and BEL1 in patterns largely resembling those seen in the integumented ovules of Arabidopsis. These genes were found to be expressed in the integument of unitegmic ovules and in the surface layers of ategmic ovules, and in some cases, expression of BEL1 was also observed in the surrounding carpel tissue. We hypothesize that ategmic ovules derive from a fusion of the integuments with the nucellus or that the nucellus has taken on some of the characteristics confined to integuments in ancestral species.

Evolutionary diversification of the flowers in angiosperms

“…Among parasites, Santalales are an interesting exception from the formation of numerous ovules per fl ower. Nevertheless, many Santalales form ovules without integuments ( Fagerlind, 1948 ;Brown et al, 2010 ). In some Santalales, reduction of ovules is so radical that they are not even formed, and thus seeds are also lacking (Loranthaceae, Balanophoraceae, Fagerlind, 1948 ).…”

Inflorescence evolution in Santalales: integrating morphological characters and molecular phylogenetics

“…Three species of Santalum with different degrees of “vegetativeness” in their inflorescences were illustrated by Stauffer (), which invites study of the expression levels of LFY across a genus with a well‐resolved species phylogeny (Harbaugh and Baldwin, ). Questions about ovular reduction in Santalales were addressed by Brown, Nickrent, and Gasser () where expression patterns for orthologs of ANT and BELL1 suggested a fusion between integuments and nucellus, not the loss of integuments in unitegmic and ategmic species. Similar evo‐devo approaches have great potential in ascertaining homology between unmodified and highly modified phenotypes.…”

Floral anatomy of the plant Psittacanthus schiedeanus (Loranthaceae)

“…In the early diverging genera Nuytsia, Gaiadendron and Atkinsonia (Narayana, 1958;Prakash, 1961), and in subtribe Elytrantheae the ategmic ovules (sensu Cocucci & Venturelli, 1982) are distinguishable in multiple locules. In the rest of the family no separate ovules are distinguishable, they apparently having become fused into a single structure called the mamelon (Brown, Nickrent, & Gasser, 2010) or "collective ovule" (Cocucci, 1983) within a single loculus. In the most recent phylogeny of Loranthaceae (Nickrent et al, 2019), Aetanthus and Psittacanthus are sister genera with strong support, but this clade is unresolved with respect to the small flowered members of subtribe Psittacanthinae.…”