Patterns of angiospermy development before carpel sealing across living angiosperms: diversity, and morphological and systematic aspects

Abstract

Almost all angiosperms are angiospermous, i.e. the ovules are enclosed in carpels at anthesis and during seed development, but angiospermy develops in different ways across angiosperms. The most common means of carpel closure is by a longitudinal ventral slit in carpels that are partly or completely free. In such carpels, the closure process commonly begins at midlength of the prospective longitudinal slit and then proceeds downward and upward. Closure by a transverse slit is rarer, but it is prominent in groups of the ANITA grade and in a few early branching monocots (some Alismatales) and some early branching eudicots (a few Ranunculaceae and Nelumbonaceae), in these eudicots combined with a more or less developed longitudinal slit. In all these cases the carpels have a single ovule in ventral median position. In ANITA lines with pluriovulate carpels, there is only a short longitudinal slit in the uniformly ascidiate carpels. In carpels with a unifacial style the closure area is narrow; this pattern is rare and scattered mainly in some wind-pollinated monocots and eudicots. In most angiosperms the carpels become closed before the ovules are visible from the outside of the still incompletely closed carpels (early carpel closure). This is notably the case in the ANITA grade and magnoliids. Delayed carpel closure, with the ovules visible before the carpels are closed, is much rarer and is concentrated in a few monocots (mainly some Alismatales and some Poales) and a few eudicots (mainly a few Ranunculales and many Caryophyllales, and scattered in some other eudicots). A kind of delayed carpel closure (with the placenta visible before closure but mostly not the ovules) also occurs in syncarpous gynoecia with a free central placenta. Most gynoecia with a free central placenta occur in the superasterids. In such gynoecia the individual carpel tips are not differentiated but the opening in young gynoecia has the shape of a circular diaphragm. In this case, when ovary septa and free carpel tips are missing, the number of carpels is sometimes unclear (Primulaceae, Lentibulariaceae, some Santalaceae). Extremely ascidiate carpels are concentrated in the ANITA grade, a few magnoliids and some early branching monocots. Aspects of potential advantages of plicate vs. ascidiate carpels with regard to flexibility of pollen tube transmitting tract differentiation are discussed.

Ancestral traits and specializations in the flowers of the basal grade of living angiosperms

“…An additional unusual feature in carpel morphology in the ANITA grade was recognized more recently (Endress, 2015). It is what happens before or when the carpel closes and how the resulting closure slit is shaped.…”

“…- Williams (2008Williams ( , 2009) has shown that pollen tubes grow relatively slowly in the transmitting tract of the carpels in the ANITA grade as compared to other angiosperms, with a speed of growth intermediate between that in gymnosperms and Mesangiospermae. Pollen tube transmitting tracts are short because long styles are lacking (Endress, 2015). However, the growth pattern of the pollen tubes, in which callose plugs are formed intermittently and keep the active part of the pollen tube constantly short even in long pollen tubes, and pollen tube growth is restricted to on or in the cell walls (without destroying the cells) or in secretion outside of the gynoecium tissue, is the same in the ANITA grade as in Mesangiospermae but differs markedly from the pattern in living gymnosperms (Willliams, 2008(Willliams, , 2009Williams & al., 2010;Taylor & Williams, 2012).…”

Pollen-Pistil Interactions and Their Role in Mate Selection

“…It serves a protective role and functions as a conduit for pollen tubes to grow to the ovary, but it also provides a venue for pollen-pistil interactions that regulate pollen tube growth and, hence, fertilization. Although the earliest diverging angiosperm lineages do not possess the full set of canonical angiosperm pistil traits (Endress and Doyle, 2009;Endress, 2011Endress, , 2015Lora et al, 2016), the core lineages (which display the most diversity) have a stigma specialized for the receipt of pollen and a style that pollen tubes must traverse before fertilization can occur. Some authors attribute angiosperm diversity to phenomena that depend on pollen-pistil interactions taking place in stigmas and styles, such as pollen competition and self-incompatibility (SI; Mulcahy and Mulcahy, 1988;Williams, 2012).…”

Floral development ofLavatera trimestrisandMalva hispanicareveals the nature of the epicalyx in theMalvageneric alliance

“…Congenital fusion : process of fusion of two organ meristems occurring from the beginning of development, in which the epidermis is not involved (Endress, ).…”