Floral development in Arabidopsis thaliana: a comparison of the wild type and the homeotic pistillata mutant

Abstract

Homeosis is sometimes defined as the replacement of one member of a meristic series by another member normally formed in a different position. The pistillata floral mutant of Arabidopsis thaliana (Brassicaceae) has petals replaced by sepal-like organs ("petals"), is male sterile, and has abnormal gynoecial development. We compared the ontogeny of wild type and pistillata flowers to determine the developmental basis for their divergent final forms. Normal sepal development in wild type pistillata flowers is indistinguishable in terms of initiation events, anatomical and morphological development, and allometric growth. Wild type petals and pistillata "petals" are initially ontogenetically similar in these same respects. The first observable difference between the two floral forms is abnormal patterns of cell division in pistillata at stamen inception. Tissues in the normal position of the androecium appear congenitally fused to the gynoecium to various extents and differentiate gynoecial cell fates. Form divergence between wild type petals and pistillata "petals" becomes evident when these organs reach 90 μm in length, after androecial developmental divergence has occurred. Pistillata does not have petals replaced by sepals; pistillata "petals" are intermediate in form between wild type sepals and petals because of the developmental switching of petal primordia into the ontogenetic pattern characteristic of wild type sepals after petal primordia are initiated.

Evolution of the Petal and Stamen Developmental Programs: Evidence from Comparative Studies of the Lower Eudicots and Basal Angiosperms

“…In support of this hypothesis, it is interesting to note that careful developmental analyses of Arabidopsis floral homeotic mutants indicate that, particularly in the first and second whorls, characters such as organ phyllotaxy and early primordium growth are genetically dissociable from organ identity (Bowman et al 1989;Hill and Lord 1989;Crone and Lord 1994;Jenik and Irish 2000). This observation may cast doubt on the reliability of such characters as indicators of independent petal derivation events.…”

AP2 Gene Determines the Identity of Perianth Organs in Flowers of Arabidopsis thaliana.

“…Wild Type Figure 1 shows a mature flower of A. thaliana (Haughn and Somerville, 1988;Komaki et al, 1988;Bowman etal., 1989;Hill and Lord, 1989) that shares common characteristics with flowers of other members of the family Brassicaceae (Jones, 1939;Vaughan, 1955;Muller, 1961;Polowick and Sawhney, 1986). It is regular, hypogynous, and consists of five concentric whorls of organs.…”

“…Organogenesis follows the typical acropetal sequence reported for the crucifer Cheiranthus cheiri (Sattler, 1973), with the exception of androecial whorls. The first primordium to appear on the floral apex is the abaxial sepal ( Figures 8A and 8B), immediately followed by the adaxial sepal and the two lateral sepal primordia (Hill and Lord, 1989). The four small petal primordia are initiated next and arise apparently simultaneously in positions alternating with sepals ( Figure 8C).…”

Branch apices, heterochrony, and inflorescence morphology in some mimosoid legumes (Leguminosae: Mimosoideae)

“…Sattler (1975b) provided a generalized discussion, and later (1978) examined heterotopy in the context of fusion and floral morphology. In none of these works is the relationship between heterotopy and heterochrony mentioned, though the idea that home os is is a result of heterochronic change is becoming widely accepted (d. Coen 1991;Hill & Lord 1989;Lord 1991;Lyndon 1994). As the taxa studied here are widely distributed on the published cladogram (Grimes 1995), little may be said about the polarity of the heterotopic change in stipule inception until a better resolved cladogram becomes available, and morphology of the stem-apex of more species is studied.…”

“…Heterotopy is the change in the position of inception of organs (Sattler 1978). Several authors argue that homeosis is a special case of heterochrony (Coen 1991;Hill & Lord 1989;Lord 1991;Lyndon 1994), and Lyndon (1994) points out that while homeotic genes are often assumed to be positional genes, their role in regulating timing (heterochrony) should be distinguished from any role in affecting position. The role of heterochrony in affecting or effecting heterotopic changes remains unstudied, and the relationships between heterochrony, heterotopy and homeosis require theoretical discussion.…”