Verne Grant

Cytogenetics of the hybrid Gilia millefoliata × achilleaefolia

SummaryGilia millefoliata andG. achilleaefolia, two annual diploid (n=9) species ofPolemoniaceae, crossed readily in certain combinations but not in others. The F1 hybrids were vigorous but sterile. They gave rise, apparently by the union of unreduced gametes, to an F2 generation of tetraploids, which were mostly fertile.Chromosome pairing in the hybrids varied markedly according to the state of nutrition of the plants. The F1 hybrids formed fewer clear diakinesis figures, fewer bivalents, fewer chiasmata per bivalent, and more attenuated or stretched bivalents when grown in 2″ pots of sand than when grown in rich soil (Table 3). A pot-bound allotetraploid individual derived from this hybrid showed the same meiotic irregularities as the starved F1s until irrigated with a solution of mineral nutrients, after which its chromosomes paired regularly in bivalents (Table 2, Fig. 38).The capacity of the F1 hybrids to produce polyploids also differed strikingly in the two cultures. The rate of polyploidy of the stunted sand-grown hybrids was 2381 viable tetraploid zygotes per million flowers, while the corresponding figure for the luxuriant field hybrids was only 2.7 per million flowers.For the production of polyploid progeny by diploid parents — a process which should be clearly distinguished from normal fertility — the termpolyploidy rate is proposed. It is suggested that starvation of a structural hybrid may sometimes increase its polyploidy rate by reducing chromosome pairing to the point where restitution nuclei and hence unreduced gametes can be formed.

Chromosome Repatterning and Adaptation

Publisher Summary Structural changes of the chromosomes affect the system of heredity, variation, and evolution in a variety of ways. The known effects of repatterning include hybrid sterility, the linkage of gene complexes, the perpetuation of heterozygotic and heterotic combinations of such gene complexes, the addition of new loci to the complement, and position effect. The deeper significance of chromosomal repatterning in many groups of organisms may reside in the relationship between the gene arrangement and the manner of functioning of the genotype itself. Chromosomal sterility would, in such cases, be a byproduct of structural rearrangements, the primary object of which was to revise the linkage relations of adaptive gene complexes, establish adaptive heterosis, add new loci or bring about a specific position effect.

The Genetic Structure of Races and Species in Gilia

Publisher Summary This chapter attempts to equate the species with the modifier complex (Harland), the interspecific genes (Lamprecht), the sterility barrier in general (numerous authors), the plasmon (Loeb), or other special aspects. Such oversimplified approaches to a complex phenomenon were bound to fail. All of the types of genetic systems that have so far discussed—the simple Mendelian factors, the polygenic systems, the specific plasmons, the sterility factors, and the karyotypes—plus the external isolating mechanisms enter into the genetical structure of species. It may not be adequate even to equate the species with the sum total of these elements, because this entity behaves as a level of integration in its own right. Yet the species does not appear to be qualitatively different in any fundamental way from the race. The same types of genetic systems and isolating mechanisms that differentiate species are also found, if not in full-fledged development, at least in rudimentary form, among races. If the species represents a higher level of organization, it is the one that can be attained by gradual processes of evolution from the level of the race.

Species hybrids and spontaneous amphiploids in the Gilia laciniata group

THE Gilia laciniata group is a complex of six interrelated species belonging to the larger assemblage of Leafy-stemmed Gilias (Polemoniace). The plants are annual herbs with small flowers which grow in sandy places in western North America and tetriperate South America (plate). They are either diploid (2n = i8) or tetraploid (2n = 36). Reproduction is mainly by autogamy. The North American species are:

The Biological Composition of a Taxonomic Species in Gilia

Publisher Summary This chapter focuses on the biological composition of a taxonomic species in Gilia. Gilia inconspicua complex a flock of sibling species related more or less independently of one another to different large-flowered species of Arachnion. This pattern of sibling species diversity within a taxonomic species is not unique in Gilia but is found in some other groups of inbreeding plants, and is expected to be of widespread occurrence. The larger assemblage of the Cobwebby Gilias or Arachnion considered as a whole, can be subdivided according to flower size into a series of large-flowered forms with corollas more than twice as long as the calyx, and a series of small-flowered forms with corollas twice or less than twice as long as the calyx. The breeding system is correlated with flower size, the showy large-flowered forms being cross-pollinated by insects to a moderate or high degree and the small-flowered plants being predominantly autogamous. The large-flowered and outcrossing Cobwebby Gilias are exemplified by such species as Gilia tenuiflora, G. latiflora, and others; while the small-flowered autogamous Cobwebby Gilias comprise the Gilia inconspicua complex. The Gilia inconspicua complex (Polemoniaceae) is a group of small annual herbs with small inconspicuous flowers that occurs widely throughout the arid regions of western North America, and has a secondary center of distribution in southern South America.

System of Classification

Perennial or annual herbs or less commonly shrubs, vines, or small trees. Leaves alternate or opposite, simple or pinnately or palmately compound. Flowers axillary or terminal, solitary or in small cymose clusters or dense heads, perfect. Calyx pentamerous, synsepalous, regular or sometimes zygomorphic, wholly herbaceous or with herbaceous lobes and membranous sinuses. Corolla pentamerous, sympetalous, regular or rarely bilabiate, aestivation contorted. Stamens 5, epipetalous, alternate with corolla lobes, the point of insertion varying, anthers 2-celled, dehiscing longitudinally. Ovary superior, inserted on a basal disk, 3-celled, rarely 2- or 4-celled, style single, stigma 3-lobed or rarely 2- or 4-lobed. Fruit a capsule, mostly loculicidal, with 1 to numerous seeds. Seeds mostly with copious endosperm and straight or slightly curved embryo, the seed coat frequently becoming mucilaginous when wet.

Systematic Position of the Family

The historical task of blocking out the major taxonomic groups has proceeded in an uneven fashion. With regard to the Polemoniaceae the first minor taxon to be recognized was a species of Polemonium and the first of the larger taxa to achieve recognition were certain of the genera. The aggregation of related families, or order, became recognized at a later stage. Within these broad limits the family gradually emerged as a recognized taxon in the last years of the 18th century and the early years of the 19th.

Problems of Classification

The Polemoniaceae is a taxonomically complex family in which generic and specific divisions have frequently been the subject of confusion and dispute. The situation was well stated by Dawson in 1936. “The Polemoniaceae, comprising [eighteen] genera and about three hundred species, while clearly defined as a natural order of plants, show unusual difficulties in the determination of generic relationships. The entire group is so closely interrelated that absolute characters, which could serve as a basis to separate genera and species taxonomically, are lacking. Since the time of Jussieu (1789) this family has presented an interesting but baffling problem for systematists. Authors have differed widely in their definition of genera. This uncertainty of status is reflected in the confusion of nomenclature existing today, as well as in the wide variations of opinions regarding the merging or segregation of genera and species.”