A functional interpretation ofLactuceae (Compositae) pollen

Pollen morphology and ultrastructure inLactuceae pollen is considered in relation to the accomodation of volume changes, pollination biology and exine-held substances. Echinate pollen grains, such as those ofCatananche, are shown to accomodate volume changes by folding along the colpi and possibly by volume changes in the cavea. The different patterns of echinolophate pollen respond in different ways. Folding along the colpi is important inScorzonera andTragopogon and to a limited extent inCichorium andEpilasia whilst inScolymus the colpi are almost immobilized. Movements of the lacunar floors take over the harmomegathic function to compensate for lack of colpus mobility. Bulging of the intine at the apertures and changes in the size of the cavea may account for part of the volume change accomodated in any pollen type. Echinolophate pollen is interpreted as being a superior means of regulating volume changes with the most economical and mechanically efficient use of wall material which has evolved independently in several tribes ofCompositae.     

Pseudointegricorpus clarireticulatum (Samoilovitch) Takahashi: morphology and ultrastructure

Dispersed tricolpate pollen of Pseudointegricorpus clarireticulatum (Samoil.) Takah. from the Upper Maastrichtian in Zeya-Bureya Basin, Amur (Heilongjiang) River area, Russian Far East/China has been studied with light and electron microscopy. Pollen size, pole outlines and the shape of equatorial projections show some variation within the species. The exine is striate-reticulate, semitectate and columellate. The species is characterised by highly complex structures that have harmomegathic function and include equatorial projections, endexinous thickenings, difference in the thickness of the infratectum, foot layer and endexine throughout the pollen grain, and equatorial furrows. Exine layers taper towards colpi regions while they break abruptly in furrow regions. The furrows could have helped to shed the exine quickly and enabled pollen germination. A non-extended region with a small cavity in the ectexine was observed in the equatorial region. We think that this region is characteristic of most Triprojectate species.     

Links between morphology and function of the pollen wall: an experimental approach

The wall of pollen grains exhibits morphological variation in many features including apertures, ornamentation and thickness, but the function of these characters remains to be clarified. It has been suggested that they are involved in the accommodation of volume changes (harmomegathy). To investigate this further, we developed a protocol that induces a controlled hydration of the pollen without affecting its metabolism and we applied it to six species differing in their pollen wall morphology. The entry of water caused pollen swelling and volume increase leading to breakage of the wall and/or of the plasma membrane, such that the per cent of intact grains was negatively correlated with the level of hydration. Qualitative and quantitative differences were observed between the species. Breakage of the exine was observed only in pollen lacking apertures and with thin exine. Variation in the exine ornamentation and thickness could explain the interspecific differences observed for the rates of breakage of the plasma membrane. Our results suggest that pollen wall morphology matters for survival and maintenance of pollen integrity further to volume increase due to hydration. We propose a rationale for future studies that should allow disentangling the contribution of different pollen morphological and physiological features to harmomegathy.     

Development of heterocolpate pollen in Myosotis scorpioides L.(Cynoglosseae,Boraginaceae)

Structure of mature pollen of Myosotis scorpioides and sporoderm development were examined using light, scanning and transmission electron microscopy. Pollen grains of M. scorpioides are heterocolpate with three colpori alternating with three pseudocolpi (pseudoapertures). Functional pseudoapertures take part in harmomegathy but not in pollen germination. Formation of colpori and pseudocolpi starts simultaneously. Structural difference between the aperture and pseudoaperture becomes clearly noticeable at the early free microspore stage when the endexine is initiated: at the colpori (at the area of ora) the endexine is composed of only a thin loose layer, at the pseudocolpi, the endexine is homogeneous and thick. Later, on the late free microspore stage, the difference appears in the structure of the intine: at the area of ora, the intine is three times thicker comparing with the pseudoapertures and mesocolpia. Besides zonal pseudoapertures, the triangular poroid areas are present at both poles of M. scorpioides pollen. Their structure and developmental pathway are similar to the zonal pseudoapertures. Pseudocolpi and polar pseudoapertures should be considered as the structures originated de novo specialised for harmomegathy.     

Pollen morphology ofSonchus and related genera,and a general discussion

The pollen morphology of the taxa belonging to the generaAetheorhiza Cass.,Launaea Cass.,Reichardia Roth andSonchus L. in the Iberian Peninsula has been studied with light and electron microscopy. The pollen is 3(-4)-zonocolporate and echinolophate (without polar lacunae, but in general with prelacunae), with equatorial ridges and 15–20 lacunae: 3–4 poral, 6–8 abporal and 6–8 paraporal. Small to medium size, P × E = 19–36 × 23–42 µm; sometimes two different sizes have been found. Exine 3–9 µm thick and ornamentation microreticulate and echinate. The results clearly show the relationships between genera. Moreno-Socías, E., Mejías, J. A., Díez, M. J., 1994: Morfología polínica deLactuceae (Asteraceae) en la Península Ibérica, I.Lactuca y géneros relacionados. — Acta Bot. Malacitana.19: 103–113.     

From acaveate to caveate: evolution of pollen types in the Rhaponticum group (Asteraceae,Centaureinae) related to extreme conditions

A survey of pollen morphology and ultrastructure within the Rhaponticum group, using both scanning and transmission electron microscopy, revealed two pollen types. Most representatives of the Rhaponticum group exhibited Serratula‐type pollen. The exceptions were the genus Oligochaeta, with Centaurea jacea‐type pollen, and the genus Myopordon, with both types of pollen. Phylogenetic inference within the group indicated that Jacea‐type pollen is derived from Serratula‐type pollen. The fact that Myopordon exhibited a series of pollen types, from Serratula type to Jacea type, suggests that a transition from one form to another has occurred in the genus. Therefore, the factor which has produced this evolutionary tendency should be sought within Myopordon. Jacea‐type pollen is characterized by a decrease in size, ornamentation, the absence of a columellar infratectum and the presence of a cavea. A comparative survey of similar tendencies in other groups, and their correlation with potential functional implications, prompted the hypothesis that the critical factor involved in such evolution of pollen could be habitats with extreme conditions. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 158 , 499–510.