Abstract
The loss of the two inner (adaxial) microsporangia (MS) on the anthers is a shared,
derived character for three species of the genus Microseris (Asteraceae). In a hybrid between M. douglasii (4 MS) and M. bigelovii (2 MS), one major gene and four modifier loci are responsible for the difference
in MS number. The homozygous recessive (2 MS) genotype of the major gene is necessary
but not sufficient for the reduction. In addition, at least five M. bigelovii (2 MS) alleles of the three major modifiers are needed for a stable 2-MS phenotype
in all florets of a plant. One, two or three M. bigelovii alleles of the modifiers cause the random reduction or loss of some of the adaxial
MS. When the major gene and two modifiers specify 2 MS and only one modifier is homozygous
for the M. douglasii (4 MS) alleles, sister plants can have any phenotype from pure 2 MS to pure 4 MS.
Here, we examine the phenotypic expression of these genotypes raised under the normal
winter annual conditions and under long-day conditions. In all cases, the phenotypes
vary among sister plants, but the range of variation (most notably under long-day
conditions) depends on the specific modifier gene contributing the M. douglasii alleles. The phenotypic variance in one of the genotypes was decreased by a factor
of ten in the depauperate heads produced in the long-day experiment. This effect is
mediated by a dependence of the MS phenotype on the position of the floret relative
to the edge of the flowering head (capitulum) and directly by the size of the capitulum.
Genotypes specifying phenotypes with more or less precisely two or four MS in all
florets show hardly any dependence on environmental or developmental factors. The
significance of these observations lies in the non-linear, “canalized” relationship
between phenotypic expression and gene dosage, which shows how a qualitative morphological
change dependent on a single major gene mutation can pass through a potentially maladaptive
intermediate stage.
Key words
Microseris
- character evolution - anther development - organ numbers - developmental canalization
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K. Bachmann
Department of Taxonomy
Institute of Plant Genetics and Crop Plant Research (IPK)
Corrensstr. 3
06466 Gatersleben
Germany
eMail: Bachmann@ipk-gatersleben.de
Section Editor: G. Gottsberger
