16. Evolution of Reproductive Organs in Vascular Plants
245
the Trimerophytophyta did not have any adjunctive organs to the sporangium. Fernsporophylls evolved in a fern lineage, while integuments, seed plant-megasporophylls (carpels in angiosperms), and seed plant-microsporophylls (stamens in angiosperms) were established in a seed plant lineage. The inner integument of the
angiosperm and the gymnosperm is thought to be homologous (Doyle 1996), but
the relationships of other reproductive organs among different lineages (e.g., the
outer integument of the angiosperm, the ovuliferous scales of conifers, the male
and female sporophylls of living cycads, the colors of Ginkgo, and the envelopes of
the Gnetales) are unclear. Although the evolutionary relationships among these
reproductive organs are not clear, the increase of organs covering the sporangium
during the course of seed plant evolution, especially in the angiosperm lineage, is
remarkable.
Evolution of morphological characters is likely to have been caused by mutations of genes, although details of the genes that cause morphological changes have
not yet been revealed because of insufficient knowledge of the molecular basis of
plant development. Recent progress in molecular developmental biology has begun
to make possible an approach to the evolution of morphological characters. In this
chapter, a hypothesis on the genetic changes that caused the evolution of reproductive organs in vascular plants is discussed.
2 Genes Related to Floral Organ Development
Molecular genetic studies using Arabidopsis and Antirrhinum have revealed that
three classes of genes (A-, B- and C-function genes) play important roles in the
development of the four kinds of floral organs: sepals, petals, stamens, and
gynoeciums (reviewed in Weigel and Meyerowitz 1994). The A- and C-function
genes manage the development of sepals and gynoecium, respectively. The combination of the A- and B-function genes and that of the B- and C-function genes
results in the development of petals and stamens, respectively. The cloning of the
A-, B-, and C-function genes revealed that most of them belong to the MADS gene
family whose members encode transcription factors (Shore and Sharrocks 1995).
The MADS genes have two well-conserved domains: the MADS and K domains.
While MADS genes have been reported from metazoans and fungi, the MADS
genes encoding the K domain are specific to the plant kingdom (Theissen et al.
1996; Hasebe and Banks 1997).
Angiosperm MADS genes have been classified into more than 10 groups (reviewed in Theissen et al. 1996; Hasebe and Banks 1997) and four of them correspond to A- (API group), B- (AP3 and PI groups) and C- (AG group) function
genes. As expected from the functions of the A-, B-, and C-function genes, they are
expressed in the first-second, second-third, and third-fourth whorls, respectively
(reviewed in Weigel and Meyerowitz 1994). Some of the other MADS genes are
also expressed in floral organs, suggesting the involvement of the genes in floral
organ development, although the function of the genes is unclear. MADS genes
245
the Trimerophytophyta did not have any adjunctive organs to the sporangium. Fernsporophylls evolved in a fern lineage, while integuments, seed plant-megasporophylls (carpels in angiosperms), and seed plant-microsporophylls (stamens in angiosperms) were established in a seed plant lineage. The inner integument of the
angiosperm and the gymnosperm is thought to be homologous (Doyle 1996), but
the relationships of other reproductive organs among different lineages (e.g., the
outer integument of the angiosperm, the ovuliferous scales of conifers, the male
and female sporophylls of living cycads, the colors of Ginkgo, and the envelopes of
the Gnetales) are unclear. Although the evolutionary relationships among these
reproductive organs are not clear, the increase of organs covering the sporangium
during the course of seed plant evolution, especially in the angiosperm lineage, is
remarkable.
Evolution of morphological characters is likely to have been caused by mutations of genes, although details of the genes that cause morphological changes have
not yet been revealed because of insufficient knowledge of the molecular basis of
plant development. Recent progress in molecular developmental biology has begun
to make possible an approach to the evolution of morphological characters. In this
chapter, a hypothesis on the genetic changes that caused the evolution of reproductive organs in vascular plants is discussed.
2 Genes Related to Floral Organ Development
Molecular genetic studies using Arabidopsis and Antirrhinum have revealed that
three classes of genes (A-, B- and C-function genes) play important roles in the
development of the four kinds of floral organs: sepals, petals, stamens, and
gynoeciums (reviewed in Weigel and Meyerowitz 1994). The A- and C-function
genes manage the development of sepals and gynoecium, respectively. The combination of the A- and B-function genes and that of the B- and C-function genes
results in the development of petals and stamens, respectively. The cloning of the
A-, B-, and C-function genes revealed that most of them belong to the MADS gene
family whose members encode transcription factors (Shore and Sharrocks 1995).
The MADS genes have two well-conserved domains: the MADS and K domains.
While MADS genes have been reported from metazoans and fungi, the MADS
genes encoding the K domain are specific to the plant kingdom (Theissen et al.
1996; Hasebe and Banks 1997).
Angiosperm MADS genes have been classified into more than 10 groups (reviewed in Theissen et al. 1996; Hasebe and Banks 1997) and four of them correspond to A- (API group), B- (AP3 and PI groups) and C- (AG group) function
genes. As expected from the functions of the A-, B-, and C-function genes, they are
expressed in the first-second, second-third, and third-fourth whorls, respectively
(reviewed in Weigel and Meyerowitz 1994). Some of the other MADS genes are
also expressed in floral organs, suggesting the involvement of the genes in floral
organ development, although the function of the genes is unclear. MADS genes