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441.
442.
D. Carl Freeman Jon Lovett Doust Ali El-Keblawy Kathleen J. Miglia E. Durant McArthur 《The Botanical review》1997,63(1):65-92
Dioecy has evolved independently, many times, among unrelated taxa. It also appears to have evolved along two contrasting
pathways: (1) from hermaphroditism via monoecy to dioecy and (2) from hermaphroditism via gynodioecy to dioecy. Most dioecious
plants have close cosexual relatives with some means of promoting outcrossing (e.g., herkogamy, dichogamy, self-incompatibility,
or monoecy). To the extent that these devices prevent inbreeding, the evolution of dioecy in these species cannot logically
be attributed to selection for outcrossing. In these cases, the evolution of dioecy is, we believe, due to selection for sexual
specialization. However, in other species, that lack outbreeding close relatives, dioecy may have evolved from gynodioecy
(males and hermaphrodites) as an outbreeding device. Subsequent disruptive selection and selection for sexual specialization
may have also shaped the evolution of dioecy from gynodioecy in these species, resulting in two genetically determined, constant
sex morphs.
Both pathways for the evolution of dioecy require the operation of disruptive selection, though the gynodioecy route involves
more restrictive disruptive selection and a genetic designation of gender. In contrast, the monoecy route is not dependent
on the genetic designation of two sex morphs, but, rather, allows the possibility of sexual intermediates and sexual lability.
Both pathways produce one morph in which maleness is suppressed and another in which the female function is negligible or
nonexistent—the reproductive mode recognized as dioecy.
Evidence is presented here to support the thesis that instances of sexual lability, the presence of an array of sexual intermediates,
sex-switching, and sexual niche segregation can be explained in terms of the pathway that was taken in the evolution of a
particular dioecious species. In addition, the degree of sexual dimorphism seen in dioecious species is correlated with mode
of pollination (insector wind-pollinated) and other ecological factors. 相似文献
443.
444.
Sequence requirements for the stimulation of gene amplification by a mammalian genomic element 总被引:2,自引:0,他引:2
HSAG-1 is a 3.4-kb genomic element from a human chronic lymphocytic leukemia--Chinese hamster ovary (CHO) hybrid cell line shown to stimulate the amplification of expression vectors in cis when transfected into a variety of cell lines [McArthur and Stanners, J. Biol. Chem. 266 (1991) 6000-6005]. Subfragments of HSAG-1 were tested for amplification activity by insertion into the vector, pSV2DHFR. The results suggest that multiple positive- and negative-acting elements were present that influenced amplification activity. The deletion of regions believed to contain positive-acting elements decreased or abolished the amplification stimulatory activity of the most active 1.45-kb fragment, supporting this hypothesis. The construction of composite sequences containing multiple positive elements and lacking negative elements, however, failed to enhance the activity; maximum activity was obtained only with the original intact configuration of elements. Two of two CHO HSAG-1-like elements tested had an activity equivalent to HSAG-1, while one of 24 random CHO genomic fragments tested had an activity as high as HSAG-1. The combination of sequence and structural features needed to affect the frequency of gene amplification may therefore be quite common in the mammalian genome. 相似文献