Phenotypic Distance Research Articles

Allomeric Variation. 2. Developmental Instability of Extreme Phenotypes

A new pattern in the variation of morphological characters is described in insects, birds, and mammals. This pattern, called the IG-OG effect for convenience, depends for its detection on distinguishing among three operational levels of variation: (1) within-individual variation in paired or homologous, segmental structures (asymmetry of bilateral organs or traits is employed here); (2) within-population variation of a trait, usually estimated by the standard deviation or coefficient of variation (CV); (3) differences in variability (CV) between different traits. The IG-OG effect (increased asymmetry in extreme phenotypes) is apparently most strongly expressed in the least variable traits (level 3), those with CVs of 2.5 or less. When the frequency distribution of such a character (level 2) is divided into three parts, the two tails (out-groups, OG) and the central part (in-group, IG), and the amount of within-individual (asymmetry) variation (level 1) is compared among them, it was found that OG individuals were more asymmetrical than IG individuals. This is the same as discovering a correlation between asymmetry and absolute distance from the mean. The IG-OG effect, including its negative correlation with the coefficient of variation, was predicted from a general theory of morphometric variation. The theory, allomeric variation (Soule 1982), assumes that, within a set of homologous or nested characters, the contribution of developmental noise (random molecular motion and interactions) to the variability of characters decreases as the size of the characters increases. As a consequence, the additive genetic contribution to the phenotype should be greatest in those traits that are the largest or have the most parts. In turn, the correlation between the genotypic homozygosity and phenotypic distance from the mean should be most pronounced in the least variable traits, except in traits, such as skull or wing measurements, encompassing two or more elements, in which integration, compensatory effects or other extrinsic processes may obscure the expression of the intrinsic genotype. If, finally, it is assumed that developmental stability is correlated with the degree of genotypic heterozygosity, then it follows that OG individuals will be less stable than IG individuals. An alternative explanation for the IG-OG effect, based on the concept of balance (coadaptation) is considered. Available information is insufficient to weigh the relative importance of these hypotheses.

Diffuse competition in Lepidoptera

THE effects of interspecific competitive encounters have traditionally been viewed in terms of species replacement, but recent workers1,2 have recognised the importance of weak or diffuse competition, which leads only to reduced niche volume. Evidence of such niche restriction has been obtained from the study of bird communities, where it has been shown that species often expand niche dimensions in areas with few species2,3. For example, one-third as many bird species are present on the island of Puercos as on the nearby Panamanian mainland and the insular birds forage in a wider range of habitat types than conspecific mainland birds4. Unfortunately this rather direct method of relating changes in niche width to changes in competitive background is often difficult to apply but the importance of competition can be evaluated in other ways. It is generally conceded that if competition occurs at all, it will be strongest among phenotypically similar species5. More explicitly, the intensity of competition which a species faces should be inversely related to its phenotypic isolation, where such isolation is a complex function of the phenotypic distance between a species and coexisting species which share limiting factors.

Variability of Different Growth Contributing Parameters of Some Okra (<i>Abelmoschus Esculentus</i> L.) Accessions and Their Interrelation Effects on Yield

The objectives of the study were to assess the variability of growth contributing characters of 50 okra (Abelmoschus esculentus L.) accessions and their interrelation effects on the yield of green pods. The experiment was undertaken at the Horticulture Farm of Bangladesh Agricultural University, Mymensingh during the period from February to May, 2002. There was a wide range of variation for spread of plant (43.73 cm), height of plant (80.90 cm) and length of petiole (12.31 cm). Moderate variation for number of nodes per plant (14.58), number of leaves per plant (24.51 at 80 DAS), length of leaf (12.20 cm), breadth of leaf (13.05 cm); and lesser variation for number of primary branches per plant (1.57) was observed. The yield of green pod varied significantly and ranged from 4.39 t/ha (accession 19) to 12.77 t/ha (accession 69) with the average value of 7.86 t/ha. Number of primary branches per plant, which showed a lesser range of variation, recorded the highest genotypic co-efficient of variation (GCV= 26.56%) and also the highest phenotypic co-efficient of variation (PCV = 32.37%). A moderate GCV and PCV were recorded in case of length of petiole (GCV = 14.24% and PCV = 15.95%), spread of plant (GCV = 12.00% and PCV = 13.06%), breadth of leaf (GCV = 9.81% and PCV = 12.41%) and length of leaf (GCV = 9.53% and PCV = 13.35%). These characters suggested the existence of justifiable genetic and phenotypic distance among different accessions. Correlation co-efficient between yield and growth contributing parameters indicated that yield of green pod had highly significant positive association with the number of nodes per plant. Path analysis of different growth contributing parameters showed that number of nodes per plant directly contributed towards the yield of green pod. Key words: Okra accessions, variability, growth parameters, GCV, PCV, path analysis. doi:10.3329/jard.v6i1.1654 J Agric Rural Dev 6(1&2), 25-35, June 2008