Lineage extinction

A mbros Less is known about how cki-1 is shut off in VPCs than about how cki-1 is turned on in these cells. However, when we misexpress lin-14 late in development, it will maintain cki-1 expression later than in the normal vulval lineage. Therefore, extinction of cki-1 expression in VPCs is influenced by some activity that is in turn dependent on lin-14 down-regulation. All our cki-1 expression constructs retain the 3hUTR of cki-1 (Hong et al 1998), and so the dynamic cki-1 expression we observe could reflect post-transcriptional regulation in addition to transcription regulation. 

In a landmark paper, Muller (1964) demonstrated that clonal lineages may be subject to reduce fitness due to a relentless increase in the load of deleterious mutations, and that sexual recombination would tend to reduce that load. The progressive increase in deleterious mutations has become known as Muller s ratchet, and there is evidence for the ratchet in nonrecombining genomes present in eukaryotic cells, namely, mitochondrial DNA (Lynch, 1997 Moran, 1996), sex chromosomes, and Drosophila chromosomes that are prevented from recombining (Rice, 1994). A potential consequence of the ratchet is extinction of clonal lineages (Lynch et al., 1993). Some models indicate that clonal species... 

The entire adaptive immune system was acquired by the immediate ancestors of the sharks (the extinct Placoderms), either by Transib retrotransposons, or by a herpesviral ancestor of the EBV lineage, or both (vide supra). Constituents of the adaptive immune system (MHC, Ragl/2, RSS, V(D)J) united first in the ancestral sharks the innate and adaptive immune systems were extensively reviewed [27, 147]. 

Multiplication of species states that there is splitting of phylogenetic lineages as well as transformational change within a lineage. Hence evolutionary change includes two processes — phyletic evolution or transformation and speciation. Some workers would include extinction as a third process and others would include extinction under phyletic evolution. This theory is also nomological-deductive. 

The character combination observed in the Powhatania flower is found mainly in Lauraceae and Hernandiaceae, but also in Monimiaceae. The paired nectariferous structures associated with the first androecial whorl are distinctive and suggest a closer affinity to Hernandiaceae and Lauraceae. The limited number of characters and specimens available preclude more detailed phylogenetic placement. Cohongarootonia shares most floral characters with Lauraceae and Hernandiaceae and may represent a separate and now extinct lineage along the stem of Lauraceae/Hernandiaceae clade. Together with other lauraceous fossils... 

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