MODERN BOTANY - P. RAVEN - 1990

SECTION IV. DIVERSITY

CHAPTER 10. CLASSIFICATION OF LIVING ORGANISMS

Sexual Reproduction

Unlike diploid eukaryotes, Bacteria do not have duplicated DNA; nevertheless, several Mechanisms of Genetic recombination are known in prokaryotes as well. As discussed in more detail in Chapter 11, a portion of their genetic material can be transferred from one Cell to another, but there is no mechanism comparable to Meiosis by which it is subsequently and regularly inherited alongside the genetic material of the recipient cell.

The only way for new Genetic information to be stably transmitted in prokaryotes is through the Integration of the acquired fragment into the bacterial DNA molecule. This process is imprecise and difficult to reproduce.

Sexual reproduction, involving the regular alternation of meiosis and syngamy, confers a high selective advantage because it serves as the primary mechanism for generating and maintaining variation in natural eukaryotic populations. As we will learn in Chapter 28, variation is a crucial prerequisite for the adaptability of living organisms to their environment during the course of evolution.

Evolution of Diploidy

The earliest eukaryotic organisms were presumably haploid and reproduced asexually; however, with the spread of sexual reproduction among them, the evolution of diploidy began. Apparently, it initially arose when two haploid Cells merged into a diploid zygote. Such an evolutionary event most likely occurred multiple times. It is believed that the zygote then immediately underwent meiotic division, restoring the haploid state (Figs. 10-10 and 10-11, A). In organisms with a simple type of life cycle, the zygote is the only diploid cell.

Class="center">Fig. 10-10. Evolution of genetic systems. Each circle corresponds to a specific type of life cycle. The most primitive eukaryotes were undoubtedly haploid for the greater part of their life cycle (the circle outlined with a single line at the bottom left). In this case, meiosis (indicated by four bold dots) occurs immediately after Fertilization (shown by the colored arrow). Other life cycles differ from the haploid one in the timing of fertilization and the duration of the diploid phase (indicated by the outer boundary of the circle). Certain modern groups of organisms characterized by the given life cycle are indicated inside the circles

"By chance" (and such a "chance event" occurred in several different evolutionary lineages), some of these zygotes divided mitotically rather than meiotically, thereby giving rise to an Organism composed of diploid cells that enter meiosis at a later stage. In animals, such delayed meiosis leads to The formation of Gametes—eggs and sperm. The gametes subsequently fuse, directly restoring the diploid state (Figs. 10-10 and 10-11, B). Thus, in animals, gametes are the only haploid cells.

Fig. 10-11. Schemes of the MAIN TYPES OF life cycles. The diploid phase is shown below and the haploid phase above the thick bar. Four white arrows indicate the products of meiosis; a single white arrow points to the fertilized egg. A. In zygotic meiosis, the zygote divides meiotically to form four haploid cells, which then divide mitotically to form new haploid cells or a multicellular individual that ultimately produces gametes through differentiation. This type of life cycle is found in Chlamydomonas and A number of other Algae. B. In gametic meiosis, the diploid individual produces haploid gametes via meiosis, which fuse to yield a diploid zygote. Division of the latter gives rise to a new diploid organism. This type of life cycle is characteristic of most animals, some protists (oomycetes), and the brown alga Fucus. C. In sporic meiosis, the sporophyte, or diploid individual, produces haploid spores through meiosis that do not function as gametes but instead divide mitotically. This gives rise to multicellular haploid organisms (gametophytes) that ultimately form gametes, which fuse to form a diploid zygote. The zygote, in turn, gives rise to diploid individuals. This type of life cycle (Morphology/12.html">ALTERNATION OF GENERATIONS) is characteristic of plants and many algae. A similar cycle is observed in the chytrid Allomyces and one closely related genus, as well as in some other protist groups not included in this book

In plants, meiosis leads to the formation of spores rather than gametes. Spores are cells capable of dividing mitotically to produce a multicellular haploid organism; in this they differ from gametes, which can only develop after fusing with one another. Multicellular haploid individuals alternating with diploid forms are characteristic of plants, certain brown, red, and green algae, two closely related genera of chytrids, and other protist groups not discussed in this book. Such a process is known as alternation of generations (Figs. 10-10 and 10-11, C). The gamete-producing haploid generation in plants is called the gametophyte, and the diploid spore-producing generation is the sporophyte. This terminology is also used for algae and occasionally applied to other groups.

In a number of algae (most red, many green, and some brown), the diploid and haploid forms are outwardly identical. Such types of life cycles are called isomorphic alternation of generations (Fig. 10-10).

There are also other life cycles in which the diploid and haploid forms are not identical. During the evolution of certain groups, Mutations occurred that manifested in only one generation, although the corresponding alleles were evidently present in both diploid and haploid individuals. In life cycles of this type, the gametophyte and sporophyte became markedly different from each other, giving rise to heteromorphic alternation of generations, which is characteristic of plants and some brown algae (Fig. 10-10).

In bryophytes (mosses, liverworts, hornworts), the gametophyte is dominant; it is nutritionally independent and usually larger than the sporophyte, which may be structurally more complex. In vascular plants, by contrast, the sporophyte is dominant, being much larger and more complex than the gametophyte, which in almost all groups depends on the sporophyte for its Nutrition.

As already mentioned, diploidy makes it possible to accumulate more hereditary information and to regulate the expression of an organism's genetic material much more precisely during its development. This is perhaps why the sporophyte in vascular plants is a large, complex, and nutritionally independent generation. One of the clearest evolutionary trends in this group, which dominates most terrestrial habitats, is the increasing dominance of the sporophyte and the suppression of the gametophyte. In flowering plants, the female gametophyte is a microscopic body of only seven cells, and the male gametophyte consists of only three cells. Both are entirely dependent on the sporophyte for nutrition.