normal sexualreproduction, genetic uniformity and hybrid vigor are lost after the F1 generation, but with apomixis these traits can be maintained through many generations due to a fixed heterozygosity ( Koltunow et al., 1995 ; Ortiz et al., 2013
Apomixis, i.e., asexual reproduction through seeds, is a natural trait of reproduction in plants ( Ozias-Akins, 2006 ). Unlike sexual seed formation, it does not involve the formation of normal female gametes or fertilization; however, it does
reproduction. The objective of this study was to estimate narrow sense heritability in guayule using a method that more accurately accounts for the contributions of apomictic and sexualreproduction.
Materials and Methods
Open-pollinated seeds, assumed to
-pollen have been largely unsuccessful (reviewed by Santamour, 1967 ). The main mode of reproduction for georgia plume is thought to be vegetative through root suckers, but plant regeneration is extremely limited. Low population numbers and a lack of sexual
; Ristaino, 1990 ). Sexualreproduction augments genetic variation in P. capsici via allelic recombination. The resulting offspring genotypes may include acquired resistance to fungicides or antibiotics, creating new challenges for phytophthora blight
fragmentation can lead to reproductive failure and even further losses ( Wetzstein and Yi, 2010 ). Genetic diversity tends to be reduced in small, isolated populations to the point where sexualreproduction may not be possible ( Ellstrand and Elam, 1993 ) in
plants required for the production of liquor, it is necessary to establish measures to promote the spread and protection of populations. Slow growth, low rate of asexual reproduction, and sexualreproduction limited by problems of pollination and seed
( Liang et al., 2005 ). This study investigated the processes of microsporogenesis and megasporogenesis and the development of male and female gametophytes. Our goal was to elucidate whether reproductive disorders exist in the sexualreproduction of
The tangerine-virescent (tv) mutation was reported as a single gene somaclonal variant from tissue culture (Evans and Sharp 1963). A replicated field trial was conducted to characterize variation and stability in the phenotype of this tv somaclone and to compare it with the inbred parent from which it was reportedly derived.
Heritability and stability of the tv somaclonal variant was measured by comparing R3 end R4 lines of sexual progenies of the original tv variant and with its sexually derived inbred parent UC82B. Several additional variants were observed in these tv lines, including fruit shape, days to first flower, fruit weight, yield, plant type, and fertility. Eight sterile or semi-sterile plants were discovered in 6 of 39 R4 lines. Our results suggest that multiple genetic changes have occurred in the tv somaclonal variant and while the original tv mutant is stably inherited, additional genetic abnormalities occur following sexual reproduction.
Guayule (Parthenium argentatum Gray) has been difficult to improve through classical plant breeding because of its facultative apomictic reproductive system. Attempts have been made to increase rubber concentration and yield by examining traits correlated with rubber production and their heritabilities. We propose a new way of estimating heritability in guayule that more accurately accounts for the contributions of apomictic and sexual reproduction. At two years of growth, there was a significant relationship between the parents and the progeny for all traits measured, except for rubber and guayulin B concentrations. Due to the facultative apomictic nature of guayule reproduction, heritabilities are more accurately presented as a range of values between the narrow and broad sense heritabilities. Since guayule is more apomictic than sexual, most heritabilities will be closer to the broad sense values. To increase resin and rubber yield in the progeny, selection should focus on height and width in the parents because height and width are highly correlated with rubber yield, with the highest heritabilities (0.65 to 1.00 and 0.97 to 1.0, respectively) of the traits measured.