What chromosomes tell us

Looking at chromosomes under the microscope is, in a way, like looking at the blueprint of life: the architect's plan rather than the finished building. This is not the place to enter deeply into the chemical and physical properties of chromosomes: a subject that attracts intense interest and research, stimulated by the ever-present desire to learn more about the origin of life. Here it must suffice to give a few simple examples to show how investigation of the number, form and pairing behaviour of chromosomes helps the classifier, the evolutionist and the plant breeder.

Many wild plants are polyploid, having two or more times the basic set of chromosomes in each cell. Diploidy is the ancestral condition: polyploidy a later derivation. Sometimes even within one species polyploidy may occur, as in the widespread Rhipsalis baccifera (4.18), which is diploid with 22 chromosomes in Brazil. Paraguay and Bolivia, tetraploid with 44 chromosomes in Mexico, Costa Rica, Cameroon, Kenya and Madagascar, and octoploid with 88 chromosomes in Madagascar also. Polyploidy has been observed within one single plant even.

a population i. This picture s the tolly ot trying to define a species from a single plant.

The large watery cells of succulent tissue in some Mesembryanthemaceae contain up to 16 times the basic chromosome number. But these cells a re not concerned with reproduction, and the plant produces normal haploid pollen and egg cells.

Chromosomes differ not only in number but also in form. Thus, in the Aloineae the basic number n=7, each set being made up of four long and three short chromosomes. Even more marked size differences are to be found in the Agava-ceae. Agave and Yucca were originally classified in separate Families (see Chapter 14), but both genera have a unique make-up: five large and 25 minute chromosomes in each set. Such a novel feature is hardly likely to have evolved twice, and this confirms the close relationship between the two genera.

In true species, the chromosomes pair regularly at meiosis, each with its corresponding partner. This pairing and subsequent uniform separation is essential for the production of good pollen and functional egg cells. In a hybrid, the two sets contain different genes, or a different order of arrangement, and pairing takes place with difficulty or not at all. In the ensuing tangle, many of the pollen grains and egg cells may contain odd numbers of chromosomes and be non-functional. This sterility of hybrids has long been recognized (the mule isa classic example), and it plays an important role in nature by deterring random crossing between allied species growing together. But few hybrids are completely sterile, and a persevering plant breeder can often persuade seeds to set if he tries hard enough. Crosses with an odd number of chromosomes—a trip-loid cactus with 33 chromosomes, for instance—could obviously never form an exact number of pairs at meiosis. A plant breeder ignorant of this could waste much time trying to hybridize, say, Medio-caclus coccineus with the epicacti. The former is tetraploid, the latter are diploids, and therefore the progeny would be sterile triploids.

Most standard botanical works of reference—monographs and floras —now cite chromosome numbers for species, if they are known.

How inheritance works To see how the chromosome cycle functions, let us take the simplest of examples: the red- and yellow-flowered Portulaca grandiflora (5.3). Red flowers are governed by gene R. yellow by gene r. A true-

Top right(5.2): The 14 chromosomes ot Bulbine alooides. magnified 1900 times. Note size variation and the apparently clear

Right(5.3): Portulaca grandiflora, showing the flower colour variation in cultivated strains of this popular annual bedding plant Flower doubling is also to be seen

Chromosome numbers

The basic sel of chromosomes, as present in pollen grains and egg cells, is haploid (n) Thus, in Aloineae n=7. in Mesembryanthemaceae n=9. in succulent euphorbias n= 10. in Caclaceae and Sfapelieae n= 11 and in Agave n=30 Each cell of the plant body contains a double (diploid) set of chromosomes, half

Higher multiples of the basic number are called polyploid. Thus:

Ruschia impressa has 2 n= 18 Diploid

Ruschia nonimpressa 2n=36 Tetraploid

Ruschia uncinata 2n=54 Hexaploid.

Haworthia tessellata 2n= 14. 28. 42 or 56 in different populations

Odd numbers (broken sets) are referred lo as aneuploid. Thus:

x Gastroleanowotnyi 2n=20 (7x3-1).

("x" before a name indicates hybfidity).

Flower colour inheritance in Portulaca

Parents

F, Generation

Pollen

F, Generation

R — Dominant gene for red flower T — Recessive gene for yellow flower RR — True-breeding red flower

Rr — Hybrid red flower (R suppresses r when they meet In one cell) rr — True-breeding yellow flower breeding red Portulaca will have the constitution RR, with two R genes at identical sites on the two corresponding chromosomes, one received from each parent. A true-breeding yellow Portulaca will have only genes for yellow, rr. In the red plant, all pollen grains and egg cells carry the single factor R; in the yellow one all carry the factor r. If we now cross a red plant with a yellow one (5.4), all the progeny will be Rr, no matter which was the male and which the female parent. This first generation, called Fi for short, will be found to have red flowers exactly like the red parent. This is because R is dominant and ris recessive: that is, when the two meet in one cell, R suppresses the action of r.

Not all pairs of genes behave in this way: sometimes the Fi shows characters intermediate between the two parents. The main point to realize is that in a hybrid the characteristics do not mingle like ink and water: each gene retains its identity and, although undetectable in one generation, may manifest itself in the next. This we shall see if we now cross our Fi red portulacas among themselves. Now we have gametes of two kinds, half R, half r. From chance combinations of pollen and egg cells, three types of progeny are possible: RR. Rr and rr. Of these, both RR and Rr will bear red flowers, and only those lacking R will bear yellow ones. By the laws of chance, then, red plants will outnumber yellow in the ratio 3:1. We call this a Mendelian ratio in tribute to Gregor Mendel, an Augustinian monk of Brünn (now Brno in Czechoslovakia), who first published the mathematical basis of inheritance in 1865'. Note thatall our second-generation (F2) plants with red flowers will look alike, and only a further crossing experiment could separate the one RR from the two Rrs. If R were incompletely dominant to r, we should be able to distinguish them visually, and we should expect the ratio 1 red: 2 orange: 1 yellow.

The science of genetics grew up at the start of the present century with the rediscovery of Mendel's work, which had been totally overlooked since its first appearance in print. His laws were found to apply universally to plants and animals. although not always in as simple a form as that set out above. One character can be governed by a number of genes (polygenes), an example being rib number and degree of white flecking in Astro-phytum. One gene may control more than one character, and instead of two there may be three or more possible states (multiple alleles) at the one site on a chromosome. If two pairs of characters

Lett (5.4): Flower colour inheritance in Portulaca. demonstrating a simple 3:1 Mendelian ratio. Knowledge ot the inheritance mechanism enables the breeder to plan his crosses and lorecast results.

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