18.6Specialization of Gametes

A particular characteristic of male gametes is that they lack most organelles other than the nucleus and cytoplasm. The same applies to spermatids in the pollen tube and mammalian sperm. This may be because the main task of male gametes is to pass nuclear DNA to the ovum, and presence of other intracellular components may adversely affect embryogenesis in the female body. It has been reported that male mitochondrial DNA is actively destroyed in the egg after fertilization. In the sperm nucleus, nuclear proteins undergo conversion and DNA is condensed during the maturation process, which inactivates the nucleus. In addition to the nucleus, sperm have a flagellum, mitochondria, and an acrosomal vesicle. All of these are devices to introduce the male nucleus into an ovum; the flagellum is a strong propellant device enabling the sperm to swim to the ovum, mitochondria are devices that supply the energy for the swim, and the acrosomal vesicle is a sac containing hydrolases that allow the sperm to penetrate the oolemma (Figure 18-11). However, these devices become unnecessary once fertilization takes place.

Ova are specialized in a number of ways. First, they are large (Table 18-1), because they contain the nutrients, organelles, protein synthesis devices, etc. needed for embryogenesis following fertilization. Second, animal ova have a zona pellucida on the surface (see Figures 18-9 and Figures 18-14). This layer physically supports the bulk of the ovum from the outside, protects it from physical damage, and functions as a species-specific barrier that only particular sperms (i.e., those of the same species) can penetrate.

In land animals and land plants, such as pteridophytes and angiosperms, reproduction is carried out between anisogametes (see Selection 1 of Chapter 2, Figure 2-4). Evolution-wise, sexual reproduction is thought to have progressed from isogamy to anisogamy, with the heterogametic degree increasing as a whole. In plants, isogamety can only be seen in algae. For instance, in asexual reproduction of chlamydomonas (Figure 18-12), two isogametic daughter cells are produced by the division of a cell. The nuclear DNA of the parent cell is transferred via the chromosome segregation mechanism, whereas the cytoplasmic DNA is transferred to the daughter cells via organelle segregation. In chlamydomonas, the organelle DNA of one parent is lost following zygosis, which affects how the cytoplasm is transferred to the zygotes and future generations. It should be understood that various types of gametes have emerged during species evolution and adaptation.

Figure 18-11 Sperm structure

Table 18-1 Ovum size

Figure 18-12 Schematic diagram of chlamydomonas

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