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Intermediates on the road to multicellularity and sexual reproduction


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A couple of other perplexing evolutionary questions:


1) How could multicellularity arise?


2) How could sexual reproduction arise?




It’s not actually all that difficult to conceptualize a solution to the first: only two main steps are needed.


1a) Unicellular organisms clump together to form a colony: each individual is autonomous and identical


1b) Some of the cells in the clump differentiate: for example, some become specialized for reproduction




A series for the second question might be more difficult to conceptualize, but we can at least cover a good deal of it.


2a) Meiosis arises (the part not explained here)


2b) Two identical haploid gametes – which swim by means of flagella - merge to form a new individual


2c) The two swimming gametes produced come to be of different sizes, but otherwise are still fundamentally alike


2d) The larger, less mobile gamete loses its flagellum and becomes immobile


2e) The large, immobile gamete becomes specialized for storing nutrients and cytoplasmic determinants, while the other, mobile gamete becomes specialized for delivering just a nucleus




And lucky for us, we can even see such intermediate forms, for both 1 and 2, in a single group of organisms!


”The Volvocaceans


The simpler organisms among the volvocaceans are ordered assemblies of numerous cells, each resembling the unicellular protist Chlamydomonas, to which they are related (Figure 2.11A). A single organism of the volvocacean genus Gonium (Figure 2.11B), for example, consists of a flat plate of 4 to 16 cells, each with its own flagellum. In a related genus, Pandorina, the 16 cells form a sphere (Figure 2.11C); and in Eudorina, the sphere contains 32 or 64 cells arranged in a regular pattern (Figure 2.11D). In these organisms, then, a very important developmental principle has been worked out: the ordered division of one cell to generate a number of cells that are organized in a predictable fashion. As occurs during cleavage in most animal embryos, the cell divisions by which a single volvocacean cell produces an organism of 4 to 64 cells occur in very rapid sequence and in the absence of cell growth.


The next two genera of the volvocacean series exhibit another important principle of development: the differentiation of cell types within an individual organism. The reproductive cells become differentiated from the somatic cells. In all the genera mentioned earlier, every cell can, and normally does, produce a complete new organism by mitosis. In the genera Pleodorina and Volvox, however, relatively few cells can reproduce. In Pleodorina californica (Figure 2.11E), the cells in the anterior region are restricted to a somatic function; only those cells on the posterior side can reproduce. In P. californica, a colony usually has 128 or 64 cells, and the ratio of the number of somatic cells to the number of reproductive cells is usually 3:5. Thus, a 128-cell colony typically has 48 somatic cells, and a 64-cell colony has 24.


In Volvox, almost all the cells are somatic, and very few of the cells are able to produce new individuals. In some species of Volvox, reproductive cells, as in Pleodorina, are derived from cells that originally look and function like somatic cells before they enlarge and divide to form new progeny. However, in other members of the genus, such as V. carteri, there is a complete division of labor: the reproductive cells that will create the next generation are set aside during the division of the original cell that is forming a new individual. The reproductive cells never develop functional flagella and never contribute to motility or other somatic functions of the individual; they are entirely specialized for reproduction. Thus, although the simpler volvocaceans may be thought of as colonial organisms (because each cell is capable of independent existence and of perpetuating the species), in V. carteri we have a truly multicellular organism with two distinct and interdependent cell types (somatic and reproductive), both of which are required for perpetuation of the species (Figure 2.11F). Although not all animals set aside the reproductive cells from the somatic cells (and plants hardly ever do), this separation of germ cells from somatic cells early in development is characteristic of many animal phyla and will be discussed in more detail in Chapter 19.


Although all the volvocaceans, like their unicellular relative Chlamydomonas, reproduce predominantly by asexual means, they are also capable of sexual reproduction, which involves the production and fusion of haploid gametes. In many species of Chlamydomonas, including the one illustrated in Figure 2.10, sexual reproduction is isogamous ("the same gametes"), since the haploid gametes that meet are similar in size, structure, and motility. However, in other species of Chlamydomonasas well as many species of colonial volvocaceans swimming gametes of very different sizes are produced by the different mating types. This pattern is called heterogamy ("different gametes"). But the larger volvocaceans have evolved a specialized form of heterogamy, called oogamy, which involves the production of large, relatively immotile eggs by one mating type and small, motile sperm by the other (see Sidelights and Speculations). Here we see one type of gamete specialized for the retention of nutritional and developmental resources and the other type of gamete specialized for the transport of nuclei. Thus, the volvocaceans include the simplest organisms that have distinguishable male and female members of the species and that have distinct developmental pathways for the production of eggs or sperm. In all the volvocaceans, the fertilization reaction resembles that of Chlamydomonas in that it results in the production of a dormant diploid zygote, which is capable of surviving harsh environmental conditions. When conditions allow the zygote to germinate, it first undergoes meiosis to produce haploid offspring of the two different mating types in equal numbers. “


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