Change in nuclear DNA content and pollen size with polyploidisation in the sweet potato (Ipomoea batatas, Convolvulaceae) complex.

Genome size evolution and its relationship with pollen grain size has been investigated in sweet potato (Ipomoea batatas), an economically important crop which is closely related to diploid and tetraploid species, assessing the nuclear DNA content of 22 accessions from five Ipomoea species, ten sweet potato varieties and two outgroup taxa. Nuclear DNA amounts were determined using flow cytometry. Pollen grains were studied using scanning and transmission electron microscopy. 2C DNA content of hexaploid I. batatas ranged between 3.12-3.29 pg; the mean monoploid genome size being 0.539 pg (527 Mbp), similar to the related diploid accessions. In tetraploid species I. trifida and I. tabascana, 2C DNA content was, respectively, 2.07 and 2.03 pg. In the diploid species closely related to sweet potato e.g. I. ×leucantha, I. tiliacea, I. trifida and I. triloba, 2C DNA content was 1.01-1.12 pg. However, two diploid outgroup species, I. setosa and I. purpurea, were clearly different from the other diploid species, with 2C of 1.47-1.49 pg; they also have larger chromosomes. The I. batatas genome presents 60.0% AT bases. DNA content and ploidy level were positively correlated within this complex. In I. batatas and the more closely related species I. trifida, the genome size and ploidy levels were correlated with pollen size. Our results allow us to propose alternative or complementary hypotheses to that currently proposed for the formation of hexaploid Ipomoea batatas.

Successive microsporogenesis affects pollen aperture pattern in the tam mutant of Arabidopsis thaliana.

BACKGROUND AND AIMS: The tam (tardy asynchronous meiosis) mutant of Arabidopsis thaliana, which exhibits a modified cytokinesis with a switch from simultaneous to successive cytokinesis, was used to perform a direct test of the implication of cytokinesis in aperture-pattern ontogeny of angiosperm pollen grains. The aperture pattern corresponds to the number and arrangement of apertures (areas of the pollen wall permitting pollen tube germination) on the surface of the pollen grain. METHODS: A comparative analysis of meiosis and aperture distribution was performed in two mutant strains of arabidopsis: quartet and quartet-tam. KEY RESULTS: While the number of apertures is not affected in the quartet-tam mutant, the arrangement of the three apertures is modified compared with the quartet, resulting in a different aperture pattern. CONCLUSIONS: These results directly demonstrate the relationship between the type of sporocytic cytokinesis and pollen aperture-pattern ontogeny.

Links between early pollen development and aperture pattern in monocots.

Although the pollen grains produced in monocots are predominantly monosulcate (or monoporate), other aperture types are also found within this taxonomic group, such as the trichotomosulcate, inaperturate, zonaperturate, di-, or triaperturate types. The aperture pattern is determined during the young-tetrad stage of pollen development and it is known that some features of microsporogenesis can constrain the aperture type. For example, trichotomosulcate pollen is always associated with simultaneous cytokinesis, a condition considered as derived in the monocots. Our observations of the microsporogenesis pathway in a range of monocot species show that this pathway is surprisingly variable. Our results, however preliminary, reveal that variation in microsporogenesis concerns not only cytokinesis but also callose deposition among the microspores and shape of the tetrads. The role played by these features in aperture pattern determination is discussed.

Multiple developmental pathways leading to a single morph: monosulcate pollen (examples from the Asparagales).

BACKGROUND AND AIMS: Early developmental events in microsporogenesis are known to play a role in pollen morphology: variation in cytokinesis type, cell wall formation, tetrad shape and aperture polarity are responsible for pollen aperture patterning. Despite the existence of other morphologies, monosulcate pollen is one of the most common aperture types in monocots, and is also considered as the ancestral condition in this group. It is known to occur from either a successive or a simultaneous cytokinesis. In the present study, the developmental sequence of microsporogenesis is investigated in several species of Asparagales that produce such monosulcate pollen, representing most families of this important monocot clade. METHODS: The developmental pathway of microsporogenesis was investigated using light transmission and epifluorescence microscopy for all species studied. Confocal microscopy was used to confirm centripetal cell plate formation. KEY RESULTS: Microsporogenesis is diverse in Asparagales, and most variation is generally found between families. It is confirmed that the whole higher Asparagales clade has a very conserved microsporogenesis, with a successive cytokinesis and centrifugal cell plate formation. Centripetal cell wall formation is described in Tecophilaeaceae and Iridaceae, a feature that had so far only been reported for eudicots. CONCLUSIONS: Monosulcate pollen can be obtained from several developmental pathways, leading thus to homoplasy in the monosulcate character state. Monosulcate pollen should not therefore be considered as the ancestral state unless it is produced through the ancestral developmental pathway. The question about the ancestral developmental pathway leading to monosulcy remains open.

Postmeiotic cytokinesis and pollen aperture number determination in eudicots: effect of the cleavage wall number.

In eudicot postmeiotic tetrads, apertures are usually joined in pairs in highly conserved areas. These appear to be located at the last points of contact persisting at the end of cytokinesis between the cytoplasm of the future microspores. In order to investigate the relationship between cytokinesis and aperture formation, aperture distribution within postmeiotic tetrads and the progression of meiosis were studied in Nicotiana tabacum cv. Ambalema. This variety (inbred line) produces about 85% tricolporate pollen and 15% tetracolporate pollen grains. In addition, about 7% of tetrads are composed of four equal-sized microspores and a supernumerary pseudomicrospore of small size and an equal proportion of tetrads exhibit unpaired apertures (these apertures are not joined in pairs within tetrads). Observation of cytokinesis indicates that both unpaired apertures and pseudomicrospores could result from the persistence of late communications between microsporocytes. Observations of tetrads indicate that an increase in the number of elements that are separated during cytokinesis is correlated with an increase in microspore aperture number. All data converge to support the hypothesis that aperture site determination is partly controlled by the number of walls formed to separate the different elements of the tetrad.

A Morphogenetic Model Accounting for Pollen Aperture Pattern in Flowering Plants.

Pollen grains are embeddded in an extremely resistant wall. Apertures are well defined places where the pollen wall is reduced or absent that permit pollen tube germination. Pollen grains are produced by meiosis and aperture number definition appears to be linked with the partition that follows meiosis and leads to the formation of a tetrad of four haploid microspores. In dicotyledonous plants, meiosis is simultaneous which means that cytokinesis occurs once the two nuclear divisions are completed. A syncitium with the four nuclei stemming from meiosis is formed and cytokinesis isolates simulataneously the four products of meiosis. We propose a theoretical morphogenetic model which takes into account part of the features of the ontogeny of the pollen grains. The nuclei are considered as attractors acting upon a morphogenetic substance distributed within the cytoplasm of the dividing cell. This leads to a partition of the volume of the cell in four domains that is similar to the observations of cytokinesis in the studied species. The most widespread pattern of aperture distribution in dicotyledonous plants (three apertures equidistributed on the pollen grain equator) can be explained by bipolar interactions between nuclei stemming from the second meiotic division, and observed variations on these patterns by disturbances of these interactions. In numerous plant species, several pollen grains differing in aperture number are produced by a single individual. The distribution of the different morphs within tetrads indicates that the four daughter cells can have different aperture number. The model provides an explanation for the duplication of one of the apertures of a three-aperture pollen grain leading to a four-aperture one and in parallel it gives an explanation for how heterogeneous tetrads can be formed.Copyright 1998 Academic Press