expressed genes in both compartments after gene transfer. This model seems inconsistent with both theory (Marshall et al., 1994; Herrman, 1997) and with empirical results (see the next section) indicating fairly rapid loss of one compartment's gene or the other after gene transfer/duplication. On the other hand, the one-loss = one-transfer model and other multiple transfer models seem unlikely considering the complex series of events required for each successful functional transfer (i.e., reverse transcription, movement to the nucleus, chromosomal integration, and functional activation, which in almost all cases requires acquisition of sequences conferring both proper expression and also targeting of the now cytoplasmically synthesized protein to the mitochondrion).
Nuclear sequences for each of three mitochondrially derived ribosomal protein genes have been reported from two separate lineages of mt gene loss as defined by our blot survey, while we have been studying the transferred rps10 gene in a number of angiosperms. These genes provide a useful starting point for investigating the number and timing of gene transfers during angiosperm evolution. In the rps14 loss lineage that includes maize and rice, the transferred gene is located within an intron of the sdh2 gene, and the sdh2 targeting sequence is alternatively spliced to rps14 transcripts (Figueroa et al., 1999a; Kubo et al., 1999), whereas nuclear rps14 in Arabidopsis shares none of these features (Figueroa et al., 1999b). Rice rps11 was duplicated in the nucleus after transfer but before targeting sequence acquisition (Kadowaki et al., 1996), with the two rps11 genes having acquired their targeting sequences from two different nuclear genes for mt proteins (atpB and coxVb), whereas the targeting sequence of pea rps11 (Kubo et al., 1998) has no similarity to any sequences currently in the databases. Finally, nuclear rps19 genes of Arabidopsis (Sanchez et al., 1996) and soybean (expressed sequence tags in the GenBank database), both rosids, also have unrelated targeting sequences and structures. The dissimilar structural features of members of each pair of these three genes strongly suggests that each was derived from a separate activation event. Because activation probably occurs relatively soon after transfer, before the nuclear gene is permanently disabled by mutation (Thorsness and Weber, 1996; Adams et al., 1999; see next section), we think that these ribosomal protein genes were not only independently activated but also independently transferred to the nucleus. Considering that rps14, rps11, and rps19 have been lost from the mt genomes of many different angiosperm lineages, as revealed by our Southern blot survey, it is possible that each gene has been independently transferred many times, not just twice. Indeed, we have recently obtained evidence for many, recent independent transfers of rps10, which has been lost from the mt genome over 20 times among the 281 angiosperms surveyed by blots (K.A., D. Daley, Y.-L.Q., J. Whelan, and J.D.P., unpublished work). It is increasingly evident, therefore, that functional transfer of mt genes to the