An expressed sequence tag (EST) analysis approach was undertaken to identify major genes involved in cold acclimation of Rhododendron, a broad-leaf, woody evergreen species. Two cDNA libraries were constructed, one from winter-collected (cold-acclimated, CA; leaf freezing tolerance -53 degrees C) leaves, and the other from summer-collected (non-acclimated, NA; leaf freezing tolerance -7 degrees C) leaves of field-grown Rhododendron catawbiense plants. A total of 862 5'-end high-quality ESTs were generated by sequencing cDNA clones from the two libraries (423 from CA and 439 from NA library). Only about 6.3% of assembled unique transcripts were shared between the libraries, suggesting remarkable differences in gene expression between CA and NA leaves. Analysis of the relative frequency at which specific cDNAs were picked from each library indicated that four genes or gene families were highly abundant in the CA library including early light-induced proteins (ELIP), dehydrins/late embryogenesis abundant proteins (LEA), cytochrome P450, and beta-amylase. Similarly, seven genes or gene families were highly abundant in the NA library and included chlorophyll a/b-binding protein, NADH dehydrogenase subunit I, plastidic aldolase, and serine:glyoxylate aminotransferase, among others. Northern blot analyses for seven selected abundant genes confirmed their preferential expression in either CA or NA leaf tissues. Our results suggest that osmotic regulation, desiccation tolerance, photoinhibition tolerance, and photosynthesis adjustment are some of the key components of cold adaptation in Rhododendron.

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Tetraploid Vaccinium corymbosum genotypes exhibit wide variability in seed set following self- and cross-pollinations. In this paper, a post-zygotic mechanism (seed abortion) under polygenic control is proposed as the basis for fertility differences in this species. A pollen chase experiment indicated that self-pollen tubes fertilize ovules, but are also 'outcompeted' by foreign male gametes in pollen mixtures. Matings among cultivars derived from a pedigree showed a linear decrease in seed number per fruit, and increase in seed abortion, with increasing relatedness among parents. Selfed (S1) progeny from self-fertile parents were largely self-sterile. At zygotic levels of inbreeding of F>0.3 there was little or no fertility, suggesting that an inbreeding threshold regulates reproductive success in V. corymbosum matings. Individuals below the threshold are facultative selfers, while those above it are obligate outcrossers. Inbreeding also caused a decrease in pollen viability, and reduced female fertility more rapidly than male fertility. These phenomena are discussed in terms of two models of genetic load: (1) mutational load - homozygosity for recessive embryolethal or sub-lethal mutations and (2) segregational load - loss of allelic interactions essential for embryonic vigor. Self-infertility in highbush blueberries is placed in the context of 'late-acting' self-incompatibility versus 'early-acting' inbreeding depression in angiosperms.

Segragation ratios at four enzyme loci (Mdh-2, Pgi-2, 6Pgd-2, and Got-3) in Vaccinium corymbosum (4x) cultivars were in close agreement with the expectations of tetrasomic inheritance. Random chromosome segregation was suggested by the absence of double reduction progeny genotypes and previous reports of bivalent pairing in this species. A common species origin of autotetraploid highbush blueberry and other Vaccinium polyploids is proposed. Different genetic and evolutionary consequences of polysomic inheritance versus disomic inheritance are discussed with reference to the literature on polyploids.

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