Embryonic and host-associated skewed adult sex ratios in dwarf mistletoe

Embryonic sex ratios were determined for the first time in dioecious flowering plants by utilizing malate dehydrogenase (Mdh-3) as a genetic marker. In three populations of Arceuthobium tsugense ssp. tsugense (hemlock dwarf mistletoe) the embryonic sex ratio was 1:1, as it also was at first reproduction. Mixed-age, adult sex ratios of these and other populations from Oregon to south-eastern Alaska, however, were significantly female-biased (59 per cent) on western hemlock, its primary host. We suggest that these female-skewed, adult sex ratios are the result of increased longevity among females. On a secondary host, noble fir, the embryonic and the adult sex ratios were both 1:1. On still another secondary host, shore pine, the embryonic sex ratio is unknown, but the adult sex ratio was significantly male-biased (55.1 per cent). Host environment apparently influences adult sex ratios in hemlock dwarf mistletoe. However, in the European mistletoe Viscum album, a gametic system apparently controls the production of the adult female-predominant sex ratios (69.9 per cent) at fertilization (or perhaps by abortion during embryogenesis). In V. album, the sex ratio of progeny raised from seed is already similarly biased at first reproduction (67.3 per cent), and the sex ratios are not influenced by host shifts.