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Breeding system, genetic variability, and response to selection in Plectritis (Valerianaceae)

University of British Columbia

Plectritis congesta and P. brachystemon are two very closely related species which grow sympatrlcally, and differ in their: breeding system, some associated morphological (floral) characters, and isozyme phenotypes. Plectritis congesta is approximately 70% outcrossed in nature, while P. brachystemon is less than 3% outcrossed in natural populations. Theory would predict that, all other things being equal, the outcrossed species would be more variable genetically than the selfed species. Since selection acts on genetic variability, the two species could be expected to respond differently to it. Six generations of plants of both species were grown under controlled conditions, and measured for a number of characters. Control and treatment (selection for tall and short height, and for early and late anthesis) populations were maintained. Two sets of P. congesta populations were maintained, one outcrossed (approximately 65%) and one selfed (outcrossed approximately 15%); the P. brachystemon populations were naturally self-pollinating. Selection pressure in the experiment was approximately 90%; 20 of the 200 plants in any population were selected to form the next -generation, on the basis of height or flowering time in the treatment populations, and at random in the control populations. The P. congesta populations responded to divergent selection for height at anthesis, indicating that genetic variability for this character was present in the populations. The outcrossed lines, PCO, diverged 66% or 148 mm from the control line; the selfed lines, PCS, diverged 78% or 175 mm. There were no significant differences between the outcrossed and selfed P. congesta lines over the course of the experiment. Two estimates of narrow sense heritability - realised heritability (b[sub=c]) and parent-offspring: regression (h²) - quantified this genetic variability: in PCO b[sub=c] = 0.53, h² = 0.45: in PCS b[sub=c] = 0.58, h² = 0.44. There was a decline in the phenotypic variance for height at anthesis in the P. corigesta lines selected for this character. In contrast, the P. brachystemon populations did not respond to selection for height at anthesis, and appear to have no detectable genetic variability for this character. Both species appear to have significant genetic variability for flowering time, as both responded to divergent selection for this character. The PCO lines diverged 33.5% or 31.8 days from the control line, the PCS lines diverged 28.7% or 27.3 days, and the P. brachystemon lines, PBS,-diverged 18.5% or 21.5 days. According to the heritability estimates, P. congesta is more variable genetically: in the PCO lines b[sub=c]=0.77, h² = 0.60; in PCS b[sub=c]= 0.75, h² = 0.72; while in PBS b[sub=c]= 0.49, and h²= 0.42. There was a decline in the phenotypic variance for flowering time in all three species groups. Of the other measured but unselected characters - number of days to emergence, number of nodes at anthesis, number of primary branches at anthesis, and fruit production - some responded to the selection pressure with divergence, notably those characters which were correlated with the selected characters (for example, number of nodes at anthesis, correlated with flowering time). With others there was no change which could be attributed to the selection procedure. There was no evidence from two qualitative characters - fruit wing phenotype and fruit pubescence pattern phenotype - for any response to selection; dispersion in both characters was not significantly different from that expected to result from random drift. The relatively high increase in aberrant characters in the P. congesta lines compared to the P. brachystemon lines is probably indicative of inbreeding depression in the normally outcrossed P. congesta. It appears that despite the difference in breeding system, the two Plectritis species are able to maintain variability by similar processes (genetic) in some characters, as in flowering time, and by different processes (genetic in P. congesta, phenotypic in P. brachystemon) in other characters, as in height at anthesis. Thus one quantitative character, height at anthesis, follows the pattern predicted by the breeding system difference, with the outcrossed P. congesta being much more variable genetically than the selfed P. brachystemon. This agrees with the levels of variability observed by Layton (1980) in electrophoretically detectable isozymes, and observed by Ganders and Maze (unpublished) in metrical fruit characters. The other quantitative character, flowering time, shows considerable genetic variance in the populations of the selfed P. brachystemon, though less than in the populations of P. congesta. The maintenance of such relatively high levels of genetic variability in the face of the strong inbreeding pressures which must be present in P. brachystemon populations is certainly adaptive, and probably comes about through occasional outcrossing and multiniche selection for variability among the segregating lines.

Text

2010-03-26

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http://hdl.handle.net/2429/22721

Botany

University of British Columbia

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