Cytogenetics, dioecism, and quantitative variation in Actinidia

Abstract

Microsporogenesis was studied in eleven Actinidia taxa including A. deliciosa (kiwifruit I and in an interspecific hyorid. From chromosome counts at meiotic rnetaphase 1 or diaKinesis, eignt taxa were classified as diploids and two as tetraploids and fu.. deliciosa was confirmed as hexaploid. A putative 2n=2x, 2n= 4x, 2n= 6x, x= 29 basis for the genus is therefore supported. Chromosomes in ail taxa were very small. Three diploids and two po1yploids analysed for meiotic pairing behaviour were all characterisea oy low cniasma frequencies per bivalent. Chromosome configurations other than bivalents were rare in A. arguta var. arguta (4xl and A. deliciosa (6x), indicating there may be genetic control of chiasmata distribution in these polyploids. The application of cytogenetic knowledge in determining ways of introducing exotic alleles into A. deliciosa is discussed. Variation in numerical and morphometric floral characters was studied in ten female genotypes, including 'Hayward' the commercial cultivar, five male genotypes and five fruiting male genotypes of A. deliciosa, all derived from the 1904 introduction to New Zealand. All female genotypes had only pistillate flowers, which contained many ovules but produced only empty pollen grains. All male vines had only staminate flowers which had high percentages of pollen grains with stainable cytoplasmic contents but which did not contain ovules. The fruiting males produced both staminate flowers and bisexual tlowers in both uniform and mixed inflorescences, and were estimated to vary from about 40% to about 70% bisexual flowers. Bisexual flowers produced high percentages of pollen grains with stainable cytoplasmic contents, showed style elongation, stigma development and ovary enlargement, and contained ovules, although pistil development did not reach tne level observed in the pistillate flowers of female vines. The three flower genders observed - pistillate, staminate and bisexual - differed in style length, ovary length, maximum and minimum ovary diameters and ovules per carpel. Staminate and bisexual flowers were similar in flowers per inflorescence, stamen filament length, pollen stainability, inflorescence rachis length and carpel number, but together differed from pistillate flowers in these characters. All flowers, regardless of gender, were similar in sepal and petal number. As well as variation among genders in floral characters, there was variation among genotypes within genders and variation among flowers within vines. For each character, estimates of the percentages of the total phenotypic variance associated with gender, genotypic and within-vine variation are presented. At the level of vine gender, a measure of Phenotypic Gender (P) is proposed, based on estimates of flower gender frequencies: P=Ef/E(f+m) where f= 1 for each flower sampled with a functional pistil and m= for each flower sampled with functional stamens. 'l'hus male ines have P = O, female vines nave P= 1, and fruiting male vines have P > 0 < 0.5. Estimates tor the tive fruiting males ranged from 0.29 to o.·41 . It is concluded that A. cteliciosa is oest describea as tunctiona1ly dioecious. In most vines, sex expression appears to be strictly controlled, leading to development of either staminate or pistillate flowers. However in rare male vines sex expression is loosely controlled, leading to development of both staminate and bisexual vines. ariation in fruit characters of the ten female and five fruiting male vines was also studied. The fruit of fruiting male vines were similar structurally to fruit of female vines, but were considerably smaller. Females and fruiting males thus differed in fruit stalk length, fruit weight, fruit length and maximum and minimum diameters, carpel number, ovules per carpel and seeds per carpel. The largest fruit from a fruiting male weighed only 35g. Low ovule number per carpel appears to be the major factor limiting fruit size in the five fruiting males studied. However, it is suggested that the variable bisexuaiity of truiting male vines is a threshold character, amenable to selection over several generations derived from controlled crosses to produce hermaphroditic cultivars. There are two features to aim for: 1.Obtaining vines with Phenotypic Gender !Pl of U.5, i.e. vines which produce only bisexual flowers.2. Obtaining cultivars with bisexual tlowers that, after self­pollination, produce fruit of commercial size, shape, appearance, taste and storage life.