EMBRYONIC AND HOST-ASSOCIATED SKEWED ADULT SEX-RATIOS IN DWARF MISTLETOE

Embryonic sex ratios were determined for the first time in dioecious f lowering 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 w as at first reproduction. Mixed-age, adult sex ratios of these and oth er populations from Oregon to south-eastern Alaska, however, were sign ificantly female-biased (59 per cent) on western hemlock, its primary host. We suggest that these female-skewed, adult sex ratios are the re sult of increased longevity among females. On a secondary host, noble fir, the embryonic and the adult sex ratios were both 1:1. On still an other 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 dw arf mistletoe. However, in the European mistletoe Viscum album, a game tic system apparently controls the production of the adult female-pred ominant sex ratios (69.9 per cent) at fertilization (or perhaps by abo rtion during embryogenesis). In V. album, the sex ratio of progeny rai sed from seed is already similarly biased at first reproduction (67.3 per cent), and the sex ratios are not influenced by host shifts.