The Embryo Lethal System

Summary

Outcrossing, wind-pollinated members of the Pinaceae have high self-pollination rates yet produce few selfed seedlings. Avoiding self-pollen capture is incomplete so how are self-pollinated ovules or seeds selectively eliminated? Barriers to selfing have long been considered to be either competition via simple polyembryony and death to selfed embryos during seed maturation. Experimental results show that simple polyembrony is a weak barrier against selfed embryos. By far, the most effective barrier is the enigmatic mechanism(s) that cause recognition and death to selfed embryos. A survey shows that extreme inbreeding depression occurs in some species but not in others so this is not a feature of conifers as a group. Only five of the 11 genera within the Pinaceae (Abies, Larix, Picea, Pinus and Pseudotsuga) have been well-characterized with respect to self-pollinated embryo deaths. Molecular dissection methods have been used to infer severity and distribution of lethal factors; to date, most are semi- lethal rather than fully lethal. These range from partially dominant to overdominant or perhaps balanced lethals.

Some selfed embryos die at all stages of seed development but a second death pattern has been detected in some Pinus and Picea spp species: a large proportion of selfed embryo deaths peak during early embryogeny. Are these dual death patterns present in other genera and if so, what genetic models might account for them? This chapter is a case study which integrates not only what was introduced in previous chapters but also shows how knowledge of the conifer mating system contributes to the broader understanding of eukaryotic systems.