2.3Cell Multiplication Through Cell Division
Predominantly Diploid Organisms
In organisms that engage in sexual reproduction, as described earlier, nuclear phases are defined by the differences in the number of chromosomes. In the case of some organisms, bodies in different nuclear phases proliferate respectively. The nuclear phase containing n is called haploid and that containing 2n is called diploid. Alternation of nuclear phases gives rise to a life cycle (Fig. 2-4). The predominant nuclear phase varies from organism to organism; organisms can have different nuclear phases in similar forms or in radically different ones. In humans, a normal individual is diploid and gametes are haploid, and therefore, the haploid state is only present during a small period of the entire life cycle. In addition, gametes exist in a form radically different from that of individual organisms. The same is also observed in seed plants. Pollen (comprising sperm cells, which are male gametes) and the embryo sac (including egg cells, which are female gametes) are haploid, but plants are generally diploid. Seeds are also diploid consisting of the cotyledon and root. In such cases, the forms of haploids or diploids are also radically different.
Fig. 2-4 The Life Cycle and Alternation of Nuclear Phases
(A) Bryophyte is mostly haploid. Diploid forms immediately spores, thus returning to haploid. (B) Individual pteridophytes are usually diploid. They are haploid only as small prothallia, which consist of archegonium (female) and antheridium (male). (C) In angiosperms, only the embryonic sac and pollen are haploid. (D) Budding yeast has two types of haploid cells, i.e., two mating types α and a, but under stressful nutritive conditions, these combine to form diploid cells. They may also proliferate as diploids, or diploids may undergo meiosis to form a tetrad of cells and then revert to haploidy. (E) Vertebrates are generally diploid, but cells of a specialized germ line undergo meiosis to form a sperm or an egg. After fertilization, an embryo is formed by the cleavage of the zygote during embryogenesis. The embryo then develops into an larva and then an adult.⇒ indicates haploids and⇒ indicates diploids. (Modified from Isao Inoue: 3-Billion-Year Natural History of Algae 2nd Ed.; Tokai University Press, beginning on page 417)
Predominantly Haploid Organisms
Non-flowering plants (or cryptogams) are slightly different. Pteridophytes, which are usually observed, are normally diploids (sporophytes) and form brown-colored sporangia around the periphery of the sporophyte leaves, which then produce multiple spores. Once dispersed, these spores germinate to form gametophytes, which are haploid and multicellular. An early developmental stage in the gametophyte is called the protonema, which consists of cells lined up in a row. The protenema soon develops into a two-dimensional prothallium, which contains the archegonium and antheridium, producing eggs and sperms, respectively. A diploid plant develops after fertilization of the egg by the sperm. Considering sporophytes as generation of asexual reproduction and gametophytes as that of sexual reproduction, alteration of generations occurs.. Thus, both haploids and diploids exist independently in pteridophytes. In contrast, bryophytes are typical haploid plants that form the archegonium and antheridium. Fertilization occurs when the sperm produced by the antheridium reaches the archegonium, and the resulting diploid cells immediately undergo meiosis, producing haploid cells. The plant body of bryophytes develops when the spore germinates, and thus, bryophytes largely consist of haploids.
Yeast is a unicellular organism comprising cells with two mating types: α and a. Under stressful nutritive conditions, the two mating types mate to produce a diploid cell. The diploid cell is capable of proliferating as diploid but also undergoing meiosis to form a tetrad of haploids that germinate to multiply as haploids (see Fig. 2-3). In this case, no great morphological difference exists between haploids and diploids. In some cases, it is difficult to morphologically differentiate the haploids and diploids that comprise a multicellular organism (e.g., seaweeds such as sea lettuce (green algae)).
In these cases, the nuclear phases are conveniently classified as haploids and diploids, but the cells are not necessarily exclusively haploid or diploid. In addition, some cells have tetraploid or octaploid phases, with an increased number of chromosomes. Moreover, the endosperm in plants is triploid. However, because the endosperm does not play a role in proliferation, it is acceptable to consider only haploids and diploids when considering the life cycle. Furthermore, plants such as seedless watermelon or banana are triploid and therefore do not produce seeds.
Discovery of Ginkgo Sperm
Column Fig. 2-2 History of the Discovery of Ginkgo Sperm
(A) The ginkgo tree in which the ginkgo sperm was discovered (in the University of Tokyo botanical garden). (B) A sketch of the ginkgo sperm (from the volume 12 of the Imperial University of Tokyo Science Bulletin). The flagellum is the coiled structure. (C) The thesis in a botanical journal first announcing the discovery of the ginkgo sperm. The usage of Katakana and Hiragana was different from that of today; the findings on ginkgo were documented in Hiragana (printed with the permission of the Botanical Society of Japan).
Everyone knows the ginkgo tree. Well, did you know that ginkgo has sperm? Did you also know that a Japanese person was the first one to discover this?
In 1896, Sakugoro Hirase, a tutor in the botanical garden of Tokyo Imperial University (the Koishikawa botanical garden) found that sperm emerged from germinating pollen found on the tips of the female flowers of the large ginkgo plant. He published his finding in an thesis in a German academic journal in 1897. In plants, including flowering plants (angiosperms), when pollen attaches to the pistils and germination occurs, the pollen tubes are elongated and the sperm nuclei move through them. In contrast, gymnosperms including Ginkgo biloba that are now well known are particularly primitive. Historically, Western scholars believed that G. biloba existed only as a fossil. The word “Ginkgo” in Latin is probably from “ginkyo” (the direct pronunciation of Japanese word for the ginkgo tree; “ginnan” in daily speech). Ginkgo exists as either a male or female tree, but only the female trees bear fruit. Female flowers blossom on female trees. However, because ginkgo is a gymnosperm, the ovule is exposed. Small droplets form on the tip and catch pollen, and the pollens germinate over several months. The sperm emerges from pollen, swims very little through water, and fertilizes the egg to form an embryo. Hirase discovered the traveling sperm in the middle of this process. In addition to ginkgo, pteridophytes and bryophytes are also plants that have no flowers, require water for fertilization, and have flagellated sperm. In angiosperms, whose sperm cells do not have a flagellum, sperm cells are led to the ovule by the growing pollen tube that contain sperm cells. In other words, what occurs in ginkgo is in the intermediate stage of adaptation to life on land. This paper was not immediately accepted by academia until the findings were confirmed by American scientists in cycads in 1897. The discovery of sperm in gymnosperms such as ginkgo and cycads was an important contribution to establishing the concepts of generational alternation in plants. Hirase’s achievements were highly appreciated and he was awarded the second Imperial Prize from the Japan Academy in 1912.
The ginkgo tree studied by Hirase can still be found in the Koishikawa Botanical Garden. It still grows well to produce ginkgo nuts. Moreover, the previous logo of the University of Tokyo was designed precisely in the shape of a ginkgo leaf in honor of the discovery. It became more schematic when the University of Tokyo was incorporated, and the new design actually differed considerably from the actual appearance of ginkgo leaves.