Pollen-tube growth pattern and chalazogamy in Casuarina equisetifolia (Casuarinaceae)

Abstract

For a better understanding of pollen-tube guidance in relation to pollen-pistil interaction, we investigated the mode of pollen-tube growth in pistils of Casuarina equisetifolia, a monoecious, wind-pollinated species that undergoes chalazogamous fertilization. The pistil is bicarpellate, but only one of the two carpels develops with two ovules. One of these ovules develops more than four embryo sacs. Pistils usually require more than 1 month to reach maturity after pollen grains have been deposited on the stigmas. During that period, pollen-tube growth proceeds discontinuously in five distinct steps that lead up to fertilization: (1) from the stigma to the upper region of the style, (2) from the upper region of the style to a septum in the ovary, (3) from the septum to the surface of the funiculus, (4) from the funiculus to chalaza in the ovule, and (5) from the chalaza to an egg apparatus. Probably because of competitive interaction between male and female gametophytes (or ovules), one pollen tube is selected from among many during the first step (just before the second step), one ovule from the two during the second and third steps, and one embryo sac from more than four during the fourth and fifth steps. On the basis of our results, erroneous drawings and explanations reported in earlier publications on chalazogamy in Casuarinaceae should be brought into question.

Pollen-tube growth and fertilization mode in Gymnostoma (Casuarinaceae): their characteristics and evolution

“…In a previous paper (Sogo et al 2004), we documented the pollen-tube growth in pistils of Casuarina equisetifolia (Casuarinaceae), and confirmed that the species undergoes chalazogamy, a mode of fertilization in which the pollen tube passes through the chalaza, instead of the micropyle (porogamy) as in the majority of angiosperms. Chalazogamy was first described by Treub (1891) in C. suberosa, and is now recognized in C. montana and C. equisetifolia (Swamy 1948;Sogo et al 2004), Allocasuarina distyla (="C. stricta") (Frye 1903), and A. nana (="C. nana") (Barlow 1958) of Casuarinaceae.…”

“…The development of ovules is retarded compared to that of the stigma and the period between pollination and fertilization is very long. Also, as in Casuarina equisetifolia (Sogo et al 2004), the pollen-tube growth is arrested in the upper region of the style until the ovary develops to a certain extent. In the case of C. equisetifolia, the pollen tube growth proceeds through five distinct steps until fertilization after pollination (Sogo et al 2004).…”

“…Chalazogamy was first described by Treub (1891) in C. suberosa, and is now recognized in C. montana and C. equisetifolia (Swamy 1948;Sogo et al 2004), Allocasuarina distyla (="C. stricta") (Frye 1903), and A. nana (="C. nana") (Barlow 1958) of Casuarinaceae. In other words, chalazogamy is known in two derived genera Allocasuarina and Casuarina in Casuarinaceae, but it is unclear whether it occurs in two other basally positioned genera Ceuthostoma and Gymnostoma (for phylogenetic relationships among the four genera, see Sogo et al 2001).…”

“…They were fixed with FAA (5 parts stock formalin, 5 parts glacial acetic acid and 90 parts 50% ethanol). Pollen-tube growth in pistils was observed by fluorescent microscopy (for methods, see Sogo et al 2004). Development of the ovary and ovules was observed using microtome sections prepared by the standard paraffin method.…”

Embryology of Cardiopteris (Cardiopteridaceae, Aquifoliales), with emphasis on unusual ovule and seed development

“…The pollen tube for fertilization enters the female gametophyte directly from the chalazal, rather than the micropylar side. Nevertheless, this is not a case of chalazogamy because, in chalazogamy, the pollen tube, passing through the lateral side of the female gametophyte, extends from the chalazal to the micropylar side as in Betulaceae and Casuarinaceae (Sogo et al 2004;Sogo and Tobe 2005).…”