Structural mechanisms of cytoplasmic inheritance in western white pine (pinus monticola dougl.)
Date
1993
Authors
Bruns, Darla Marie
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Abstract
The structural mechanisms governing cytoplasmic inheritance are described for Pinus monticola Dougl. and are consistent with patterns of cytoplasmic inheritance determined by molecular studies for some members of the Pinaceae. Ultrastructural observations support paternal plastid inheritance and predominantly-maternal mitochondrial inheritance with the possibility of occasional, low-level, paternal mitochondrial contribution. Although not demonstrated in this study, the possibility of occasional, low-level, maternal plastid contribution is not excluded.
The mature egg cytoplasm of P. monticola is organized into three concentric zones; the perinuclear, mid- and peripheral zones. The egg nucleus is central and is surrounded by a narrow, perinuclear zone dominated by egg-cell mitochondria. Outside this zone is a mid-zone which contains numerous small inclusions, scattered organelles and, occasionally, large inclusions. All egg-cell plastids exist as large inclusions which are mostly limited to the peripheral zone of egg cytoplasm and thus are isolated from the perinuclear zone. Body-cell plastids and mitochondria are transferred into the egg with the two male gametes at fertilization. Although small numbers of these body-cell organelles are sometimes carried to the egg nucleus with the fertilizing male gamete, most body-cell plastids and mitochondria remain clustered together and follow some distance behind the fertilizing male gamete. Body-cell organelles which accompany the fertilizing male gamete may escape lysis through their incorporation into the perinuclear zone. The proembryo neocytoplasm which originates the embryo cytoplasm is formed by the melding of the perinuclear zone with the nucleoplasm released during coenocytic divisions of the zygote nucleus. Paternal mitochondria outside the perinuclear zone appear to degenerate, but clustered body-cell plastids remaining in the egg cytoplasm are not affected and are incorporated into the neocytoplasm probably during migration of proembryo free-nuclei to the chalazal pole of the egg. It is possible that some large inclusions may occasionally be introduced into the neocytoplasm prior to tier formation in the proembryo.