Meiosis in a Haplo-Diplontic
Life Cycle
(Campbell 6th Ed. Fig. 13.5; 7th Ed. Fig. 13.6)
The haplodiplontic life cycle is characteristic of plants and some algae. For our puposes, we will be studying meiosis as it occurs in the microsporangium (pollen chamber) of the anther of a flowering plant, the lily. The anther is the part of a flowering plant which produces the pollen. We will be covering this in more detail later in the year. For now just understand that this a location in plants were meiosis occurs.
Sections through the anther reveal the microsporangia where meiosis takes place. Immature microsporangia contain microsporocytes which undergo meiosis (note that the suffix -cyte in meiosis is applied to a cell which will undergo meiosis). Microsporocytes are diploid.
Click to view microsporocytes in metaphase I of meiosis.
At this time homologous chromosomes, each composed of two chromatids, come to be adjacent to one another and intertwine (synapse). In these tetrads genetic exchange may occur between homologs in a process called crossing over (See Campbell 6th Ed. 244; 7th Ed. 248).
Subsequent to synapsis, the homologous chromosomes separate during anaphase, with one member moving to each pole of the division spindle. Since this is a separation involving entire chromosomes, and not chromatids, the chromosome content of the cells at the end of meiosis I has been "reduced" from the diploid to the haploid condition.
Click to view cells in Anaphase II of meiosis
In meiosis II the haploid number of double-stranded chromosomes separate at the centromere and the resulting single-stranded chromosomes move to the poles. As in mitosis, the chromatids when separated from each other are called daughter chromosomes. At the spindle poles, each group of daughter chromosomes becomes enclosed in a nuclear membrane. At the end of meiosis II each microsporocyte has been converted to a tetrad of haploid microspores.
Click to examine the lily anther cross sections showing a tetrad of microspores
Click to view a tetrad of microspores
Each microspore differentiates into a mature pollen grain.
