THE ROLES OF POLYEMBRYONY AND EMBRYO VIABILITY IN THE GENETIC SYSTEM OF CONIFERS

The mating and regeneration habits of many important coniferous species seem conducive to self pollination, exchange of pollen among relatives in small family groups, and relatively high levels of inbreeding. Most species show large inbreeding depression in growth, and the frequency of self seedlings in wind-pollination progeny is generally low. In this paper, I will show that low embryo survival plus the presence of more than one embryo in a single ovule (polyembryony) allows considerable post-fertilization embryo abortion and selection without comparable wastage of ovules. In combination they form an important mechanism for maintaining heterozygosity as well as local adaptation. Self incompatibility, in the sense of blockage to pollen tube penetration and selective fertilization (Squillace and Bingham, 1958), could perform a similar function. It also would contribute to maintaining heterozygosity in the presence of high frequencies of self pollination. Experimental evidence for self incompatibility in conifers is lacking (Hagman and Mikkola, 1963; Hagman, 1972) but it can not be ruled out at this point. If selective fertilization occurs it could further augment the effect of polyembryony. Low viability of inbred embryos, a feature of many of the widespread coniferous species, is attributed to recessive lethal and deleterious alleles which become homozygous after inbreeding (Orr-Ewing, 1957; Sarvas, 1962; Hagman and Mikkola, 1963; Mergen et al., 1965). Many of these alleles apparently are present at low frequencies in the breeding populations (Koski, 1971). Cytological examinations have shown embryo abortion to occur early in embryogeny after apparently normal pollen germination and fertilization (OrrEwing, 1957; Hagman and Mikkola, 1963; Mergen et al., 1965; Sarvas, 1968). Polyembryony, a rare derivative phenomenon in angiosperms but unusually common in gymnosperms (Chamberlain, 1966, p. 348), is of two types. One is categorized as simple, archegonial, polyzygotic, or non-cleavage, and the other as monozygotic or cleavage. Archegonial polyembryony, characterized by independent fertilization of more than one archegonium within an ovule, gives rise to heterogenic embryos within the ovule. Within the family Pinaceae, it occurs in the genera Larix, Picea, and Pseudotsuga (Dogra, 1967, p. 17). Monozygotic polyembryony is characterized by cleavage of the embryo and gives rise to isogenic embryos. Both monoand polyzygotic polyembryony occur in the genera Pinus and Tsuga (Dogra, 1967, p. 17, 83). The situation is not clear in Abies, which is classified as having non-cleavage polyembryony by Dogra (1967, p. 16); but in Abies amabilis at least there appear to be more embryos than could arise from noncleavage polyembryony alone (Owens and Molder, 1977), which indicates a pattern like Pinus and Tsuga. Polyembryony eliminates the effects of aborted embryo development when the abortion occurs in an ovule that contains an additional functional embyro to replace the degenerating one. Also, because one embryo usually ends up dominant in the embryo cavity, polyembryony provides opportunity for selection between embryos of unequal vigor during early embryo development. Embryological development can be climatically as well as genetically disturbed. Monozygotic polyembryony has the potential for reducing the effects of climatic disturbance; archegonial polyembryony has the potential for reducing