Katsuei Yonezawa

The effective size of mixed sexually and asexually reproducing populations.

Using the transition matrix of inbreeding and coancestry coefficients, the inbreeding (NeI), variance (NeV), and asymptotic (Neλ) effective sizes of mixed sexual and asexual populations are formulated in terms of asexuality rate (δ), variance of asexual (C) and sexual (K) reproductive contributions of individuals, correlation between asexual and sexual contributions (ρck), selfing rate (β), and census population size (N). The trajectory of NeI toward Neλ changes crucially depending on δ, N, and β, whereas that of NeV is rather consistent. With increasing asexuality, Neλ either increases or decreases depending on C, K, and ρck. The parameter space in which a partially asexual population has a larger Neλ than a fully sexual population is delineated. This structure is destroyed when N(1 –δ) < 1 or δ> 1 –1/N. With such a high asexuality, tremendously many generations are required for the asymptotic size Neλ to be established, and Neλ is extremely large with any value of C, K, and ρck because the population is dominated eventually by individuals of the same genotype and the allelic diversity within the individuals decays quite slowly. In reality, the asymptotic state would occur only occasionally, and instantaneous rather than asymptotic effective sizes should be practical when predicting evolutionary dynamics of highly asexual populations.

Morphology, fertility and cross-compatibility of somatic hybrids between Brassica oleracea L. and B. campestris L.

Abstract Thirty-one somatic hybrids between a heat tolerant cabbage cultivar ‘Yoshin’ ( Brassica oleracea var. capitata L.) and a Chinese cabbage cultivar ‘Kenshin’ ( Brassica campestris var. pekinensis L.) were obtained with the aim of breeding new Brassica vegetables adapted to tropical areas. After excluding two hexaploid somatic hybrids, 29 individuals were investigated for pollen fertility and seed set after self-pollination and back-crossing with the parental species. All 29 amphidiploid somatic hybrids showed pollen fertility higher than 70%, and 28 set seeds by self-pollination (2.2 seeds per silique). The hybrids were cross-compatible with Chinese cabbage (1.5 seeds per silique), while practically incompatible with cabbage. Progeny lines obtained by self-pollination had larger leaves than, and intermediate leaf shape of, the parental species. The lines obtained by back-crossing with Chinese cabbage showed larger leaves than the parents and similar leaf shape to Chinese cabbage. These progeny lines, on the whole, exhibited an appreciably wide variation in both leaf size and shape. Variation was found also within the lines for leaf shape. The somatic hybrid method, therefore, could provide new genetic variations to be used to develop new vegetable cultivars.

Formulation and estimation of the effective size of stage-structured populations in Fritillaria camtschatcensis, a perennial herb with a complex life history.

The effective population size (Ne) is formulated based on a stage‐structured population model and is estimated for two populations of Fritillaria camtschatcensis (L.) Ker‐Gawl. (Liliaceae), a perennial, mainly clonally reproducing herb. Plants in these populations change life‐history stages year by year, either upward or downward across three unambiguously identifiable stages: one‐leaf, nonflowering; multileaf nonflowering; and multileaf, flowering stages. Plants of all stages produce clonal progeny (bulblets) each year, and death of plants occurs only in the first stage. The populations are nearly at equilibrium in both population size and stage structure. Ne is estimated to be 20‐30% of the census population size (N), leading to the prediction that a population size of about 20,000 or more will be needed to conserve the normal level of the gene diversity (Ne≥ 5000). With the current demographic pattern of this species, accelerated growth of the first‐stage plants with reduced survival of the second‐ and third‐stage plants will increase both the annual (Ny/N) and generation time (Ne/N) effective sizes of population.

A theoretical basis for measuring the efficiency of selection in plant breeding

A new index of selection efficiency, i.e. the ratio of the probability of achieving the desired genetic gain in a target population to the cost expended for that gain, is introduced to define the optimum selection procedures that give the most opportunities for producing new commercial varieties. Monte Carlo simulations of mass selection showed that the index gives solutions to some important optimization problems which cannot be solved based on the traditional indices, the expected genetic gain or its ratio to the cost. The optimum cycle and intensity of selection and optimum population size for a target population could be defined by the new index, but not by the traditional indices.

Effectiveness of some management procedures for seed regeneration of plant genetic resources accessions

Procedures for seed regeneration of plant genetic resource accessions were investigated in terms of their effect on the variance effective population size and the probability that the initial allelic diversity is maintained after 10 and 20 cycles of regeneration. Four regeneration systems were compared: a bulk system (BL) where seeds are collected and treated in bulk, a partial sampling system (PS) where seeds are collected from not all, but some plants in the population with an equal number of offspring being raised from each sampled plant, a single seed system (SS) where accessions are regenerated so that each plant leaves one progeny, and the biparental mating system (BP) of Gale & Lawrence (1984) where plants are pollinated in pairs with one offspring being raised from each of the paired plants, or two offspring from one of the paired plants. It was shown that the relative efficiency of the four systems largely depends on the rate of selfing and that differences in the effective population size of the systems increase with increasing rates of selfing. The SS system gave by far the largest effective population size in regenerating the seed of moderately or highly selfing species. Although the BP system gave the largest effective size for outcrossing species, the SS system, when combined with selfing, gave a much larger effective size. The BL and PS systems were in no case the most effective. Of these two, PS system with a sampling fraction of 50% was as effective as BL, but less effective with a sampling fraction smaller than 50%. Calculations of the maintenance of the allelic diversity, however, revealed that differences between the systems are not appreciably large unless the accessions are regenerated over 10 or more cycles with 50 or fewer plants.