Haploid cells created by meiosis do not immediately become gametes such as ova and sperms. As an example, in angiosperms, a female haploid cell undergoes three more divisions to create eight cells, one of which becomes an egg cell (Fig. 12-9A). Through a single division, a male haploid cell becomes a reproductive cell (generative cell) and a vegetative cell to support a generative cell, and the generative cell is further divided into two cells (Fig. 12-9B). The two resulting sperm cells later fertilize an egg cell and a central cell of the female, respectively (double fertilization). Interestingly, in lower plant forms such as moss, the generation time of haploid cells is very long, while that of diploid cells is very short (Fig. 12-10).
Compared with plants, higher animal forms such as mammals have a very short haploid generation time. In mammals, although meiosis for oogenesis is initiated in the early stages, the process is arrested at the primary oocyte stage in the prophase of meiosis I (Fig. 12-11). In humans, the process then remains dormant for many years. Once individuals mature and hormone secretion is initiated, meiosis is resumed and ova are rapidly formed. Unfertilized ova are then promptly removed.
In mammalian males, spermatogenesis is initiated after sexual maturation. In humans, it takes 24 days for a spermatocyte to complete meiosis and become four spermatids, and approximately 9 weeks for a spermatid to become a mature sperm (Fig. 12-12). Unlike ova, much of the differentiation process for sperms occurs after they become haploids. Sperms compensate for the disadvantage of being haploids by forming a special structure called a syncytium*4. In other words, a spermatogonia does not undergo cytokinesis during the first somatic cell division and the subsequent meiosis, and the resulting cells continue to share the cytoplasm. Haploid spermatids therefore inherit the cytoplasm from diploid cells, and this cytoplasm controls the differentiation of sperms. The syncytium also synchronously contributes to spermatogenesis.
Syncytium: a coenocyte created by the fusion of multiple cells that share the cytoplasm.
Fig. 12-9A. Gametogenesis in plants
Female gametes. The figure shows a pattern common to many angiosperms. A megaspore mother cell is divided into four haploid cells by meiosis I and II. Of these cells, only one matures into a megaspore. Through three mitoses, this megaspore becomes an embryo sac consisting of eight cells.
Fig. 12-9B. Gametogenesis in plants
Male gametes. A pollen mother cell undergoes meiosis to become a pollen tetrad, which becomes dissociated and produces four microspores. The nucleus of each microspore moves to the side wall before mitosis I. This mitosis has unequal cell division, producing a large vegetative cell and a small generative cell having a nucleus with condensed chromatin structure. The generative cell moves into the vegetative cell and divides into two spermatids via mitosis II.
Fig. 12-11. Oogenesis in mammals
A primordial germ cell that moves into an ovary in early embryogenesis, becomes an oogonium. After performing several mitoses, the oogonium starts meiosis I and becomes a primary oocyte. In mammals, primary oocytes are formed in the very early stages, and their development is arrested in the early stage of the first division until the individual becomes sexually mature. Once this happens, a small number of cells periodically mature under the influence of hormones, complete meiosis I to become secondary oocytes, and become mature ova via meiosis II. During this process, two polar bodies are released. The stage at which ova are released from the ovary for fertilization differs by species.
Fig. 12-12. Spermatogenesis in mammals
Progeny cells derived from the same spermatogonium are connected through the cytoplasmic bridge until they are differentiated into mature sperms. The structure is called syncytium. To aid understanding, the figure shows how two connected spermatogonia become eight connected haploid spermatids through meiosis. The actual number of connected cells simultaneously differentiated through meiose is much higher than shown in the figure.
Agrobacteria and Genetically Modified Plants
It was discovered in 1974 that the agrobacteria-related swelling in plants is caused by the circular DNA of bacteria. Subsequent studies showed that part of this circular DNA is incorporated into the plant genomic DNA and is replicated along with DNA replication. It was also found that the inserted DNA contains plant hormone synthesis genes that promote the growth of plant cells, as well as synthesis genes for special amino acids that bacteria feed on. Indeed, bacteria cause the host plant to produce large amounts of plant cells on which they feed. In other words, bacteria use the host plant as a factory to produce their food. Based on these findings, this system was proposed for use in artificially introducing various genes to plant cells. It is currently common practice to introduce only target genes to plant cells via agrobacteria by removing the genes that cause swelling. One somatic plant cell can be directly differentiated to form a whole plant; this ability is known as totipotency, and makes it easy to regenerate a plant from a plant cell with introduced genes. The plants with artificially introduced genes are called transgenic plants. Many transgenic plants have already been created, and crops with a pest-resistance gene as well as pesticide-resistant plants are widely cultivated. Since environmental destruction, including desertification, is predicted to progress in the future, the creation of genetically modified crops that can be grown under poor conditions is an urgent issue.