Gametic Genotypes Research Articles

Early Microspore Divisions and Subsequent Formation of Microspore Calluses at High Frequency in Anthers ofHordeum vulgareL.

The products of first microspore mitosis were studied in anthers of Hordeum vulgare. These anthers were obtained either directly from plants at the binucleate stage or from tillers which had been estimated visually as containing pollen at the mid-uninucleate stage and then pretreated by being clipped off at ground level and allowed to stand in water for 2 d. In microspores from the latter plants ten-fold increases in the failure of nuclear differentiation, alteration of the axis of division, or both were observed. High frequencies of further microspore response in individual anthers were subsequently induced through culture of the spikes from the pretreated tillers in agitated liquid medium. A five to ten-fold increase in the percentage of anthers producing macroscopic callus was observed in these spikes when compared with the controls. It is suggested that nucleo-cytoplasmic disturbances in the microspores, brought about through the pretreatment period, stimulated the further divisions which eventually resulted in microspore callus formation. Microspore calluses were produced most often following an equal first division, although when mitosis resulted in differentiated nuclei further divisions of the vegetative nucleus also resulted in callus production. Haploid, diploid, polyploid, and mixaploid microspore calluses were produced but no evidence was obtained to link haploid production with any particular developmental pathway. Macroscopic calluses which emerged from the cultured anthers were probably mixtures of cell populations derived from several or many different gametic genotypes.

MARGINAL POPULATIONS OF CEPAEA NEMORALIS (L.) ON THE BRENDON HILLS, ENGLAND. II. VARIATION IN CHIASMA FREQUENCY.

In different species genetic recombination and the release of variation may be regulated in different ways and at different levels of organization since the components of the control system do not necessarily act in the same direction (e.g., Grant, 1958). For example, in herbaceous plants short generation times promote the release of variability and are often associated with low chromosome numbers: a restrictive device (Grant, 1958). The recombination system within a population may similarly be adjusted so that there is a balance between immediate fitness and long-term adaptability (Carson, 1955a). In populations of an outbreeding organism an important level of genetic control may be through the regulation of chiasma frequency since, given sufficient heterozygosity, a raised chiasma frequency will lead to an increase in the range of gametic genotypes. Differences in chiasma frequencies between populations are therefore possibly adaptive, perhaps reflecting differing ecological circumstances under which these populations exist. In Drosophila the frequency of inversion heterozygotes, in which crossing over is restricted, is important in determining the level of recombination within a population. In certain species, particularly D. robusta and D. willistoni, the amount of inversion heterozygosity decreases as the margins of the species' ranges are reached. In a series of papers (da Cunha et al., 1950; Townsend, 1952; da Cunha et al., 1959) the view was advanced that the degree of chromosomal polymorphism is a function of the variety of ecological niches in the territory which a population inhabits. Carson (1955a, b, 1956, 1964), however, suggested that greater chromosomal homozygosity at the margins serves to allow more recombination which is necessary to provide sufficient flexibility for adjustment to suboptimal conditions. He has also shown (Carson, 1958) that the response to selection in strains originating from marginal populations is greater than that in strains originating from central ones, perhaps reflecting the occurrence of more recombination in the former populations. A completely different situation occurs in the grasshopper Myrmeleotettix maculatus. The presence of B chromosomes has been shown to raise both the frequency and the variance of chiasmata in those individuals which contain them (Barker, 1960; Hewitt and John, 1967). The frequency of B-containing individuals varies between populations and it has been shown that this variation is related to climatic factors such as temperature and rainfall (e.g., Barker, 1966, Hewitt, 1973). Central populations in favorable (warm, dry) environments possess more B chromosomes and have higher chiasma frequencies than more marginal populations in stringent (colder, wetter) localities which have no B chromosomes. A comparable situation may occur in the grouse locust Tettigidea lateralis where Fontana and Vickery (1973) found that the effect of B chromosomes in a marginal population was to change chiasma position with the net effect of depressing recombination. Cepaea nemoralis (L.) has a haploid number of 22 chromosomes (Perrot, 1938). There is a large metacentric pair forming the A group, a medium sized pair, the B

The genetics of the mammalian gamete.

The genetics of the mammalian gamete.

DOUBLE AND SINGLE BACKCROSS LINKAGE ESTIMATES IN AUTOTETRAPLOID MAIZE

INKAGE estimates in autotetraploids are more complicated than in diploids for three reasons outlined by FISHER (1949) : 1) The multiplicity of segregating genotypes-for IWO loci with two alleles at each there are 19 doubly heterogenic genotypes in an autotetraploid, compared with only two in a diploid. 2) The eleven possible modes of gamete formation, compared with only two in diploidstetraploid gametes may contain all combinations of recombinant and non-recombinant chromatids, whereas diploids may be only recombinant or non-recombinant. 3) The frequencies of different gametic genotypes cannot be identified by a single backcross to a tester. In this study, for example, where the linked genes were shrunken and waxy on chromosome 9, doubly dominant progeny of the first backcross could have been produced by five gametic genotypes,ShWx/ShWx, ShW.x/Shwx, ShWx/shWx, ShWx/shwx, and Shwx/ shWx. Three of the gametes had recombinant chromatids, but all gametes were indistinguishable in the first backcross. The gametic series may be determined, however, if these progeny of the first backcross are subjected to a second backcross. Each gamete is then recognized, not by the appearance of one individual, but by the frequency distribution observed in a second-backcross family from that individual. FISHER’S monumental paper (1947) set forth the general theory of polysomic linkage and accounted for all the complexities. He then applied this theory to tetrasomic linkage anabysis in Lythrum salicaria (1 949), using double-backcross data. The second backcross, although laborious, is an extremely powerful tool in unravelling the complex gametic segregation and providing unbiased recombination estimates. The time and space required to raise and identify the progeny of a second backcross are almost inmx”ntab1e problems in higher plants. Hence, most linkage estimates in autotetraploids have been computed from single-backcross data. These include DEWINTON and HALDANE (1931) with Primula, SANSOME, (1933) with Solanum, and WELCH (1962) with maize. Single-backcross linkage can be estimated comparatively easily using maximum likelihood ( MATHER 1938). However, a single backcross cannot directly identify the recombinant

Equilibria in Auto-Tetraploids under Natural Selection for a Simplified Model of Viabilities

where p' and q' represent the gene frequencies in the next generation. Such systems have been examined in detail by Fisher [1930]. When tetrasomics are considered, the recurrence relations analogous to these will involve not gene frequencies but gametic genotype frequencies. If there are two alleles, say G and g, there are three possible gametic genotypes GG, Gg, and gg, whose respective frequencies may be denoted by u, v, and w respectively such that