The evolution of fertilization modes independent of the micropyle in Fagales and `pseudoporogamy'

In contrast to a majority of angiosperms showing porogamous fertilization, several fagalean families such as Betulaceae and Casuarinaceae are known to show chalazogamy, where fertilization is effected by a pollen tube passing through the chalaza instead of the micropyle. Our developmental study of pollen-tube growth in pistils of Myrica rubra (Myricaceae, Fagales) further shows that pollen tubes reached the nucellus before the micropyle is formed by the integument. Since fertilized ovules appeared as if the pollen tube had passed through the micropyle for fertilization, we propose the new term `pseudoporogamy' to this mode. By mapping diverse modes of fertilization, dependent or independent of the micropyle, onto a phylogenetic tree of Fagales, it appears that fertilization mode evolved from porogamy to chalazogamy and then further from chalazogamy to pseudoporogamy. Possible reasons for the evolution of fertilization modes independent of the micropyle in Fagales are discussed.