The role of prezygotic events in the reproductive success of conifers

The purpose of this study was to determine if any prezygotic breeding barriers exist in conifers. Prezygotic events were analyzed for both in vivo and in vitro systems in conifers. The events that were monitored included archegonial development, pollen germination, pollen tube growth and penetration of female structures and gamete delivery.;Aspects of archegonial development in vivo were verified by serial sections of glycol methacrylate embedded sections. This study describes novel abnormal archegonial structures in Dunkeld larch. Normal archegonial development was also investigated in Dunkeld larch and Douglas-fir. This study demonstrates that the ratio between egg nuclear volume and egg cell volume in conifers is consistent with the linear relationship that is known to exist between cell and nuclear volume in all other land plants. In contrast to other plant cell nuclei, most of the DNA in the egg cell nuclei of conifers is localized at the periphery of these organelles. In addition, the relationship between DNA content and nuclear volume in conifer egg cells is not consistent with the linear relationship that is shown between nuclear DNA content and nuclear volume in angiosperms.;This study also examined the interactions between pollen and ovules as well as interaction of pollen and female gametophytes in vivo to determine if there were any prezygotic barriers to foreign pollen in conifers. Dissections of Dunkeld larch and Douglas-fir ovules that had been pollinated with either Dunkald larch, Douglas-fir, western white pine or Interior spruce pollens revealed that heterospecific pollen has a reduced capacity to germinate in these ovules. Serial sections of glycol methacrylate embedded specimens showed that the nucellus also posed a barrier to some foreign pollen. Western white pine pollen was unable to penetrate the nucellus of either larch or Dougals fir. However, Douglas-fir pollen was able to penetrate not only the nucellus of larch ovules, but also megagametophytes and egg cells into which it delivered gametes. Larch pollen was also observed to penetrate Douglas-fir nucelluses and megagametophytes, though no gamete delivery was observed.;In vitro co-culture was also used to study the interaction of pollen and megagametophytes of different genera. Cells of megagametophytes provided no barrier to pollen tubes, and pollen tubes were able to penetrate any part of megagametophytes. Delivery of gametes was confirmed between spruce and larch. This study demonstrated that the megagametopbyte plays no role in male selection.;To study the effects of culturing on megagametophytes of Douglas-fir and larch, cones were collected at the time of fertilization and the megagametophytes were removed and then placed on medium. A variety of cell types proliferated including prothallial, neck and jacket cells. Some of these multiplying cells showed a binucleate condition. This was the first report of neck cell multiplication and induction of a binucleate state for gymnosperm megagametophyte cells in vitro.;This study demonstrates that a number of prezygotic events can influence reproductive success in conifers.