GAILING, OLIVER* and KONRAD BACHMANN. Institute of Plant Genetics and Crop Plant Research (IPK), Correnstrasse 3, 06466 Gatersleben, Germany. - Evolutionary reduction of inner pollen sacs (microsporangia) within the asteracean genus Microseris: a model system for the evolution of diagnostic characters.
Reduction from the normal 4 pollen sacs (microsporangia) to 2
microsporangia (MS) per anther has occured independently in about 50
angiosperm families and thus is a prime example for the parallel
evolution of a potentially diagnostic character (homoplasy). Within
the asteracean genus Microseris the reduction of the inner
(adaxial) MS is a synapomorphy for three diploid annual species. The
cosegregation of MS numbers with genetically mapped molecular markers
(AFLPs) was analyzed in the F6 inbred population of an interspecific
cross between tetrasporangiate (4MS) M. douglasii and
disporangiate (2MS) M. bigelovii. One major gene and four
modifying genes specifically affecting the expression of the inner MS
were detected. The dominant (4 MS) allele of the major gene was
epistatic over three of the modifiers. The modifiers could stabilize a
2 MS phenotype only in the homozygous recessive (2 MS) genotype of the
major gene. 2 MS were produced if at least 5 alleles of those
modifying genes determined the 2 MS phenotype. Homozygosity for the 4
MS allele in only one of the modifying genes in an otherwise 2MS
genetic background produced an unstable phenotype with variable
average MS numbers in different plants. However more than two alleles
from M. douglasii (4 MS) led to the production of 4 MS in most
flowers of one capitulum. This non-linear ("canalized")
relationship between gene dosis and expression of MS and the
observation of genotypes with variable expression of inner MS suggests
how the disporangiate phenotype has evolved from a stable
tetrasporangiate phenotype with a minimum of potentially maladapted
intermediate phenotypes.
Key words: diagnostic characters, Microseris, microsporangia, organ numbers, QTL