Michael S. Zavada

Comparative morphology of monocot pollen and evolutionary trends of apertures and wall structures

Data on pollen aperture and wall ultrastructure are reviewed for the monocots. New ultrastructural data on 30 taxa representing 18 monocot families are also presented. The evolutionary trends of apertures and pollen wall structure are discussed and it is proposed that the evolutionary trends of pollen apertures and wall structure in the monocots parallel those proposed by Walker for the dicots. However, the importance of these trends differ for monocots, suggesting that the selective pressures affecting pollen wall and aperture evolution in dicots and monocots have been similar, but with different emphasis.SommaireDonnées expérimentales sur l’ouverture de pollen et la ultrastructure du mur des Monocotylédones sont revus. Les nouvelles données expérimentales sur 30 taxa, représantantes 18 familles de Monocotylédones sont aussi presentés. Les cours évolutionnaires des ouvertures et structure du mur de pollen sont discutés et c’est proposé que les cours évolution-naires des ouvertures de pollen et structure du mur en Monocotylédones se mettent en parallèle de ceux proposes par Walker pour les Dicotylédones. Toutefois l’importance de ces cours sont différents pour les Monocotylédones, donnent que la selection naturelle touchant le mur de pollen et les cours évolutionnaires des ouvertures en Dicotylédones et Monocotyledones ont été semblables mais avec des accentuations différantes.АбстрактКонспект. Однодольны е обозреваются на дан ные ультраструктуры пыльцевой апертуры и стены. Новые ультраст руктурные данные 30 таксонов, представля ющих 18 родов однодольн ых, также представлены. Обсуждаются эволюци онные направления ап ертур и структуры пыльцевой стены, и предлагается, что эволюционные направления пыльцев ых апертур и структур ы стены для однодольных параллельным тем, кот орые Валкер предлага л для двудольных. Однако, важность этих направлений различа ется для однодольных в том, что отборные дав ления, влиявшие на эво люцию пыльцевой стены и апертуры для д вудольных и однодоль ных, были похожи друг на друга, но отлич ались в важности.

Pollen Wall Development of Austrobaileya maculata

Pollen wall development of Austrobaileya maculata was investigated from the initiation of pollen mother cells to anthesis. Pollen wall development can be divided into three phases: premeiotic, tetrad, and free spore. The ektexine develops within the tetrad phase and follows the pattern for other tectate-columellate angiosperm pollen thus far investigated. The endexine develops in the free-spore phase and has two modes of deposition, depending on whether it is apertural or nonapertural. Intine development begins prior to the completion of the endexine and becomes interlayered with the endexine, causing the endexine to corrode when acetolyzed because of the solubility of the intine. The last major developmental event is the injection of tapetal substances into the interstices of the ektexine. At anthesis the pollen is binucleate and has fully developed ektexine (with tapetal substances occupying its interstices), endexine, and intine.

Pollen of the Asteraceae from the Paleocene-Eocene of South Africa

Two species of dispersed pollen (Tubuliforidites antipodica and T. viteauensis) that have affinity with the Asteraceae were recovered from Paleocene-Eocene sediments of South Africa. This is the earliest unequivocal record of the Asteraceae. The two species were examined with light, scanning, and transmission electron microscopy. The pollen wall structure of both taxa are of the Anthemoid type, a pollen wall structural type considered to be primitive among asteraceous pollen. The wall structure type is indicative of many taxa of the Mutisieae. The appearance of this pollen type in the Paleocene-Eocene of South Africa supports a early Tertiary, west Gondwana origin of the Asteraceae, and the evolutionary basal position of the Mutisieae.

Mesozoic continental vertebrates with associated palynostratigraphic dates from the northwestern Ethiopian plateau

The East African Rift separates the northwestern and southeastern Ethiopian high plateaus, which are capped by massive Cenozoic volcanics overlying thick deposits of marine and nonmarine Mesozoic sediments. During geological mapping projects of the 1920s-1930s, a few Mesozoic vertebrate fossils were found on the southeastern plateau. in contrast, beginning in 1976, and then from 1993 to the present, paleontological field work in the Abay (Blue Nile) River gorge along the eastern edge of the northwestern plateau resulted in the discovery of fossil chondrichthyans (Priohybodus, Hybodus, Rhinobatos), osteichthyans (Lepidotes, cf. Pycnodus), dipnoans (Asiatoceratodus), chelonians (Pelomedusidae, Plesiochelyidae, Pleurosternidae), crocodylians (Goniopholis), dinosaurs, (cf. Acrocanthosaurus Hypsilophodontidae), pollen and other microfossils documenting a coastal biota in part, if not entirely, of latest Jurassic (Tithonian) age. These fossils include new biogeographic records for Africa and document biostratigraphic range extensions. The Ethiopian Mesozoic fauna adds to the growing evidence of limited interchange of vertebrates between Africa and Western Europe during the transition from the Jurassic into the Cretaceous.

The Role of Self-Incompatibility and Sexual Selection in the Gymnosperm-Angiosperm Transition: A Hypothesis

Self-incompatibility is viewed as a mechanism by which the female can evaluate the quality of the male gametophyte. This does not preclude its function as an outbreeding mechanism, but rather expands its role as a means to mediate the quality of the offspring by the female. The early angiosperm self-incompatibility system is envisioned as consisting of stigmatic recognition, with an incompatible combination resulting in carpel abortion. The morphological transition from the gymnospermous to the angiospermous condition (i.e., the placement of sterile sporophytic tissue between ovules and pollen) occurred concomitantly with a change in the timing of mate assessment (postzygotic to prezygotic). The selective impetus for the evolutionary development of the closed carpel is hypothesized to be the origin of self-incompatibility rather than solely a means of protection. This shift in timing prevents mechanisms that were effective for reducing male competition in gymnosperms and intensifies male competition in angiosperms (Mulcahy 1979, 1981). Thus, strong selective pressure was exerted on the male to develop alternative mechanisms for increasing its chances of acceptance by the female in angiosperms. The evolutionary development of accessory floral parts and the morphological modification of these parts to influence insect behavior might be viewed as male mechanisms to better ensure more-efficient transfer of pollen to a receptive female, thus improving its chances of acceptance by the female (cf. Willson and Burley 1983). The evolution of various insect-pollination syndromes, especially those involving faithful pollinators, is considered one of the most important events to have influenced the morphological diversity subsequently observed in angiosperms in the Cretaceous (Crepet 1979, 1983, 1984, and references therein; Mulcahy 1979).

Pollen morphology of Ulmaceae

Abstract Thirty-two species from 17 genera were examined by scanning electron microscopy and representatives from each subfamily were examined by transmission electron microscopy. The treatment of Ulmaceae based on pollen grains is consistent with the treatment of the family presented by I. A. Grudzinskaya. Wall structure and exine sculpturing are the best diagnostic pollen characteristics for determining subfamilial affinities. The pollen morphological characteristics of the Celtidoideae are more like those of Moraceae than of Ulmoideae, thus lending support to Grudzinskayas suggestion that the Celtidoideae should have familial status and occupy a phylogenetically intermediate position between the Ulmaceae (Ulmoideae) and the Moraceae.

The relation between pollen exine sculpturing and self-incompatibility mechanisms

In angiosperm pollen the reticulate-perforate exine sculpturing is associated with sporophytic self-incompatibility (S.S.I.) and imperforate and microperforate exine sculpturing is associated with gametophytic self-incompatibility. The earliest unequivocal angiosperm pollen conforms to exine morphology of pollen from plants with S.S.I. The orgin of S.S.I. is hypothesized to have coincided with the appearance of what is now the earliest recognizable angiosperm pollen. Other angiosperm characteristics correlated with S.I., functional stigmatic areas, large showy flowers (or aggregated inflorescences), and passive seed dispersal, provide some insight into the biological aspects of the earliest angiosperms.

The role of clothing fabrics as passive pollen collectors in the north‐eastern United States

The purpose of this investigation is to determine if clothing fabrics act as passive pollen collectors, and to determine if different fabrics vary with regard to the abundance and type of pollen trapped. Five of the most common fabrics in the United States (cotton, wool, polyester, silk and linen) were used to trap pollen. The pollen collecting apparatus was constructed of a 30 cm diameter circular needlepoint hoop, which vertically rotated freely, and was mounted on a dowel that was driven into the soil to chest height. Five pollen collectors, each with one of the five fabrics were placed at a collection site in rural, suburban, and urban habitats in Rhode Island for a 24 h period at weekly or biweekly intervals throughout 2002–2003. Pollen was washed from each of the fabrics and acetolysed. Total pollen per cm2 removed from each of the fabric types was estimated using a haemocytometer. The pollen types were identified, and 200 grains were counted to determine the relative abundance of the various pollen types recovered from the fabrics. Clothing fabrics act as passive pollen collectors and the flora recovered from the fabric represent the qualitative and quantitative components of the pollen rain for that specific day. There are quantitative differences among the relative abundance of pollen types from the three habitats (urban, suburban, and rural). Washing with water and a detergent eliminates a majority of the pollen from the fabrics.

Lygodium Foliage with Intact Sorophores from the Eocene of Wyoming

Specimens of Lygodium kaulfussi with attached sorophores and in situ spores from the late Eocene Bridger Formation of Woming are described and compared with similar fossil remains from the early Tertiary of other areas in North America, Europe, South America, and Australia. On the basis of the same criteria emphasized in the taxonomy of extant Lygodium species, the Eocene material from Europe and North America represents a single wide-ranging species that was distinct in some features from the Eocene species known from the Southern Hemisphere. Although not identical with any modern species, L. kaulfussi closely resembles extant L. palmatum of eastern North America in having highly dissected nonlaminar fertile pinnae and psilate spores. Strong dimorphism between the sterile and fertile foliage, as occurs in L. kaulfussi and L. palmatum, was widespread in both the Nothern and Southern Hemispheres in the Eocene and predates the Neogene appearance of the weakly dimorphic foliage characteristic of most extant Lygodium species.

The Ultrastructure of Pollen Found in the Dispersed Sporangia of Arberiella (Glossopteridaceae)

Dispersed sporangia of the Arberiella type were isolated from Upper Permian sediments from Hammanskraal, South Africa. The sporangia are morphologically similar to those attached to the scale leaves of Eretmonia, microsporangiate reproductive organs believed to have glossopterid affinity. Pollen isolated from the dispersed sporangia is monosulcate, saccate with 4-7 taeniae on the corpus that are parallel to the long axis of the pollen grain. Pollen wall ultrastructure is alveolar in the saccus regions. In the corpus region the infrastructural layer is composed of irregular shaped rods or partitions. The wall structure of pollen found in the sporangia of Arberiella differs from that found in extant gymnosperms by lacking a complete separation of the sexine and nexine in the saccus region. In a paleolandscape dominated by wind-pollinated gymnosperms, sacci may be a morphological feature of pollen that reduces male-male competition and prevents foreign pollen from occupying the micropylar space that would prevent access to the female by the conspecific pollen. All sacci increase pollen size without requiring an increase in male resource allocation in each pollen grain. This is accomplished in saccate pollen grains with a considerable amount of diversity in shape, size, number, and wall structural characteristics of saccate pollen.