Ecological and phylogenetic constraints determine the stage of anthetic ovule development in orchids.

PREMISE: Unlike most flowering plants, orchid flowers have under-developed ovules that complete development only after pollination. Classical studies reported variation in the stage in which ovule development is arrested, but the extent of this variation and its evolutionary and ecological significance are unclear. METHODS: Here, we used light microscopy to observe ovule development at anthesis for 39 species not previously studied and surveyed the literature gaining information on 94 orchid species. Tropical and temperate members of all five orchid subfamilies as well as species with contrasting pollination strategies (rewarding versus deceptive) and life forms (epiphytic versus terrestrial) were represented. We analyzed the data using statistical comparisons and a phylogenetic generalized least square (PGLS) analysis. RESULTS: Apostasioideae, the sister to the rest of the orchids, have mature ovules similar to other Asparagales, while under-differentiated ovules are present in the other subfamilies. Ovule developmental stages showed high variation even among closely related groups. Ovules were more developed in terrestrial than in epiphytic, in temperate than in tropical, and in rewarding than in deceptive pollination orchid species. This latter comparison was also significant in the PGLS analysis. CONCLUSIONS: These results suggest that ovule developmental stage in orchids can be shaped by ecological factors, such as seasonality and pollination strategy, and can be selected for optimizing female reproductive investment.

Stomatal development in the cycad family Zamiaceae.

BACKGROUND AND AIMS: The gymnosperm order Cycadales is pivotal to our understanding of seed-plant phylogeny because of its phylogenetic placement close to the root node of extant spermatophytes and its combination of both derived and plesiomorphic character states. Although widely considered a 'living fossil' group, extant cycads display a high degree of morphological and anatomical variation. We investigate stomatal development in Zamiaceae to evaluate variation within the order and homologies between cycads and other seed plants. METHODS: Leaflets of seven species across five genera representing all major clades of Zamiaceae were examined at various stages of development using light microscopy and confocal microscopy. KEY RESULTS: All genera examined have lateral subsidiary cells of perigenous origin that differ from other pavement cells in mature leaflets and could have a role in stomatal physiology. Early epidermal patterning in a 'quartet' arrangement occurs in Ceratozamia, Zamia and Stangeria. Distal encircling cells, which are sclerified at maturity, are present in all genera except Bowenia, which shows relatively rapid elongation and differentiation of the pavement cells during leaflet development. CONCLUSIONS: Stomatal structure and development in Zamiaceae highlights some traits that are plesiomorphic in seed plants, including the presence of perigenous encircling subsidiary cells, and reveals a clear difference between the developmental trajectories of cycads and Bennettitales. Our study also shows an unexpected degree of variation among subclades in the family, potentially linked to differences in leaflet development and suggesting convergent evolution in cycads.

The effect of seasonality on developmental stages of anthetic ovule integuments in Mediterranean orchids.

Orchids, differently from most flowering plants, have under-differentiated ovules at anthesis that require pollination to complete differentiation. This ovule developmental stage has been often observed in tropical species in which the absence of an evident seasonality may allow plants to extend their phenology beneficiating of a long time for post-pollination events. Here, we used scanning electron microscopy (SEM) to detect ovule integument developmental stages in 21 species of Mediterranean Orchidoideae and Epidendroideae and in 11 tropical Epidendroideae with the aim of understanding whether species with a seasonal constraint and shorter time for post-pollination ovule maturation are characterized by different stages of ovule development at anthesis. We found that Mediterranean orchids (both Orchidoideae and Epidendroideae) have more developed ovule integuments than tropical orchids. Most species show partially to fully developed ovules at anthesis with the exception of Cephalanthera where ovules are arrested in a very early developmental stage. Despite the phylogenetic signal, anthetic ovule integument development differs between related species (with different pollination strategies or blooming times), suggesting the presence of some ecological constraints. The synchronization between ovule integuments and megagametophyte development, as found in tropical orchids, is also confirmed in Mediterranean orchids. Our data show that Mediterranean and tropical orchids clearly differ in anthetic ovule developmental stages, likely depending on seasonality.