John Parker

Male and female flowers of the dioecious plant sorrel show different patterns of MADS box gene expression.

Male and female flowers of the dioecious plant sorrel (Rumex acetosa) each produce three whorls of developed floral organs: two similar whorls of three perianth segments and either six stamens (in the male) or a gynoecium consisting of a fertile carpel and two sterile carpels (in the female). In the developing male flower, there is no significant proliferation of cells in the center of the flower, in the position normally occupied by the carpels of a hermaphrodite plant. In the female flower, small stamen primordia are formed. To determine whether the organ differences are associated with differences in the expression of organ identity genes, cDNA clones representing the putative homologs of B and C function MADS box genes were isolated and used in an in situ hybridization analysis. The expression of RAD1 and RAD2 (two different DEFICIENS homologs) in males and females was confined to the stamen whorl; the lack of expression in the second, inner perianth whorl correlated with the sepaloid nature of the inner whorl of perianth segments. Expression of RAP1 (a PLENA homolog) occurred in the carpel and stamen whorls in very young flower primordia from both males and females. However, as soon as the inappropriate set of organs ceased to develop, RAP1 expression became undetectable in those organs. The absence of expression of RAP1 may be the cause of the arrest in organ development or may be a consequence.

Chromosome-specific control of chiasma formation

A desynaptic mutant of Hypochoeris radicata, 2n = 8, has been found in a population from France. The mutant is remarkable in that at metaphase-I over 90% of PMCs have a pair of univalents while two pairs are found in only 1% of cells. Only chromosome IV is affected by the desynapsis which is controlled by a single recessive gene. Bivalent chiasma frequency in cells with a pair of univalents is higher than in cells with complete bivalent formation indicating partial dependence of cell chiasma number on the availability of some factor within the anther. The F2, produced by full-sib mating, has a raised frequency of univalents in the chromosomes other than pair IV. The higher level results from enforced sib-mating of an obligate outbreeder and is not related to the action of the major gene. It is suggested that chiasma formation is controlled both polygenically and by major genes which operate on a hierarchical system. Some genes affect the behaviour of the entire complement while the action of others is specific to individual chromosomes.

The B-chromosome system of Allium schoenoprasum

Nine morphologically distinct euchromatic B-chromosomes have been identified in Allium schoenoprasum from the River Wye, South Wales. The most common type (89%) is telocentric (Bt−1) and it is likely that the non-standard Bs are derivatives of Bt−1 by deletion, centric shift and/or centric misdivision. New B-types have also been produced from standard Bs in controlled crosses. In general, the Bs are mitotically extremely stable, although occasional plants, particularly those carrying non-standard Bs, are conspicuously variable in their B-constitution between root-tip cells. In addition, B-chromosome number is enhanced in some anthers of about one third of plants. Behaviour of B-chromosomes during meiosis is described. Although there is little bivalent formation, less than 5% of the Bs are lost during meiosis in anthers. There is, however, no evidence of B-chromosome accumulation in the offspring of controlled crosses, usually a slight loss, and Bs have deleterious effects on aspects of vigour and fertility. Thus, no satisfactory explanation for populations with up to 65% B-containing individuals has yet been found.

Chiasma frequency effects of structural chromosome change

Three structural chromosome changes in the plant Hypochoeris radicata 2n = 8 have been tested for their effects on chiasma formation: (1) centric fission of chromosome 1, (2) a whole arm exchange between chromosomes 1 and 3, and (3) an interchange between the long arm of chromosome 1 and the short arm of 2 which gives an effectively three-armed pachytene multiple. Mean chiasma frequencies were compared between full-sibs in families segregating for the rearrangements. In each family the chiasma frequency was higher in heterozygotes than basic homozygotes. The size of the chiasma increase is dependant on the number of additional potentially-paired segments in the complement at pachytene. Fission heterozygotes and 1/2 interchange heterozygotes, with one extra pairing region, both form about 0.45 more chiasmata per PMC than full-sib basic homozygotes. The 1/3 exchange, with two additional pairing regions, increases chiasma frequency by twice this, about 0.85 per PMC. Individuals homozygous for the centric fission maintain the raised chiasma level. The chiasma increase appears limited to the chromosome(s) affected by structural change with no detectable interchromosomal effect.

Aneuploidy and isolation in two Hypochoeris species

The annual species Hypochoeris glabra (2n = 10) and the perennial H. radicata (2n = 8) hybridise readily in nature and in experiment. During meiosis in F1 hybrids the maximum association is a chain of seven and a bivalent indicating that at least three interchanges differentiate the two genomes. The nucleolar chromosomes in the two species are homologous and form a ring bivalent. They are, however, differentiated since in the F1 hybrid only one nucleolar-organiser region is expressed. Although chromosomal differentiation reduces the egg fertility of F1 hybrids to about 1%, viable backcross hybrids to H. radicata as pollen parent have been experimentally produced and occur in natural populations. Backcrosses with 8, 9 and rarely 13 chromosomes are found and those with 2n = 8 are fully interfertile with H. radicata. Gene flow may therefore take place in natural populations across an aneuploid barrier. The direction of gene flow in Hypochoeris is probably unidirectional from the annual to the perennial.