Effective Pollination Period and Parentage Effect on Pollen Tube Growth in Apple

Flower receptivity is a limiting factor for the fertilization of several tree fruit. The effective pollination period (EPP) can be used to determine flower longevity and identify limiting factors by assessing stigmatic receptivity, pollen tube growth rate, and ovule longevity. EPPs were determined for three apple cultivars under natural field conditions in Washington State in 2019 and 2020. In addition, a greenhouse study, performed under semi-controlled conditions, evaluated the influence of six maternal parents on the pollen tube growth performance of six pollen sources. The duration of the stigmatic receptivity ranged from 6.3 to 8.1 days, depending on the cultivar and year—pollen tubes required between 5.5 and 7.0 days from the stigma to reach the ovules. Ovule longevity of non-pollinated flowers varied between 8.2 and 11.3 days. Combinations of these factors resulted in EPPs ranging from 3.0 days for ‘Rubinstar’ to 5.6 days for ‘Olsentwo Gala’ in the present experimental conditions. The greenhouse study revealed that parentage affected pollen tube growth performance. Importantly, a significant interaction between maternal and paternal factors indicated that the performance of different pollen sources depended on the maternal parent and that general recommendations on pollination need to account for the maternal parent.

A Research on the Determination of Phenological and Molecular Characterization in Open-pollinated Genotypes in Walnut

This research was carried out to reveal the phenological and genetic differences between the S-1/1 walnut genotype and 94 F1 genotypes obtained from this genotype with each other and with the maternal parent. In the phenological observations made, it was observed that bud burst in genotypes took 52 days, leafing 50 days, leaf yellowing 31 days, and defoliation date 27 days. When the maternal parent (S-1/1) and the genotypes were compared, it was found that there was a phenological variation of 75.54 % in budburst, 73.41 % in the leafing, 34.05 % in leaf yellowing, and 93.62 % in defoliation date, while the average variation was 69.15 %. In molecular genetic analyzes, 7 ISSR primers were used to determine genetic variations, as a result, 7 monomorphic and 45 polymorphic bands were obtained, and the rate of polymorphism was found to be 86.53 %. The average number of alleles was calculated to be 7.42. In genotypes, the polymorphism information content (PIC) value varied between 0.48 and 0.95, while the average PIC value was calculated to be 0.73. As a result of cluster analysis, it was seen that genotypes were divided into 2 main clusters and 2 subsets. At the end of the study, it was determined that the S-1/1 walnut genotype and F1 genotypes obtained from this genotype have a significant variation both phenologically and genetically.

Development and Verification of an Interspecific Hybridization Protocol for Asclepias

Milkweed (Asclepias sp.) is an important pollinator genus across North America and is a host plant for many butterfly species, notably the monarch butterfly (Danaus plexippus). Commercial production of Asclepias is limited to a few species, because most species lack commercial traits, with minimal branching habit, excessive height, and minimal color variation. This study used a commercially viable Asclepias species, butterfly weed (Asclepias tuberosa L.), as a maternal parent and trialed three different pollination methods in an attempt to create interspecific hybrids. Pollination methods included a traditional method, a pollen–solution-based method, and a novel inverted pollinia method. The inverted pollinia method increased pollination success rates 4-fold among intraspecific crosses of A. tuberosa. When pollination methods were optimized, A. tuberosa was used as the maternal parent, and one-way crosses were made to seven other Asclepias species using the inverted pollinia method. Of the seven species used as pollen donors, four developed hybrid seed successfully: green milkweed (Asclepias hirtella Woodson), purple milkweed (Asclepias purpurascens L.), showy milkweed (Asclepias speciosa Torr.), and common milkweed (Asclepias syriaca L.). As germination methods vary significantly among Asclepias species, three methods of germination were trialed on seed developed via interspecific hybridizations: direct seeded, cold–moist stratification, and embryo rescue. Of the three methods, cold–moist stratification was superior to direct seeding and embryo rescue. This research is the first documented case of a controlled interspecific hybridization event among these species.

Transcriptome profiling and comparison of Rhinanthus major and Rhinanthus minor reciprocal F1 hybrids during seed stratification and germination

Germination is a vital stage in a plants life cycle, and a different germination behavior of offspring in comparison to their parents can have fitness consequences. In studies on hybridization between Rhinanthus minor and R. major, low germination rates of F1 hybrids with R. major as the maternal parent have often been reported. In contrast, the F1m hybrid, with R. minor as the maternal parent, germinates readily and rapidly. In order to find the cause of this difference, we used RNA-Seq to obtain transcriptome profiles of F1a and F1m seeds during stratification at 4C and just after germination, after 40 days of stratification for the F1m seeds and 60 days for the F1a seeds. A comparison of the transcriptome of F1a seeds that had just germinated (60 days) with non-germinated F1a seeds after 40 and 60 days revealed 2918 and 1349 differentially expressed (DE) genes, respectively. For F1m seeds, 958 genes showed differential expression in germinated and non-germinated seeds after 40 days. The DE genes of F1a and F1m hybrids clustered into two separate groups, even though they had the same parents, and no differentially expression was found for plastid genes. Non-germinated F1a seeds had an abundance of enzymes and proteins associated with peroxidase activity, peroxiredoxin activity and nutrient reservoir activity. Expression of genes related to seed germination and seed development increased in non-germinated F1a hybrid seeds between 40 and 60 days of cold stratification. F1a seeds that had germinated showed an upregulation of genes related to the gibberellic acid-mediated signaling pathway and response to gibberellin, along with a low expression of DELLA superfamily. Although the results demonstrated strong differences in gene expression during stratification between the reciprocal hybrids, we could not identify its cause, since no plastid genes were differentially expressed. It is possible that differences in embryo development after seed formation and before stratification play a role, including epigenetic imprinting.

Single nucleus analysis of Arabidopsis seeds reveals new cell types and imprinting dynamics

Seeds are the basis of agriculture, yet their full transcriptional complexity has remained unknown. Here, we employ single-nucleus RNA-sequencing to characterize developing Arabidopsis thaliana seeds, with a focus on endosperm. Endosperm, the site of gene imprinting in plants, mediates the relationship between the maternal parent and embryo. We identify new cell types in the chalazal endosperm region, which interfaces with maternal tissue for nutrient unloading. We further demonstrate that the extent of parental bias of maternally expressed imprinted genes varies with cell cycle phase, and that imprinting of paternally expressed imprinted genes is strongest in chalazal endosperm. These data indicate imprinting in endosperm is heterogeneous and suggest that parental conflict, which is proposed to drive the evolution of imprinting, is fiercest at the boundary between filial and maternal tissues.

Genomic identification of intergeneric hybrids in New World wood-warblers (Aves: Parulidae)

The documentation of hybrids between distantly related taxa can illustrate an initial step to explain how genes might move between species that do not exhibit complete reproductive isolation. In birds, some of the most phylogenetically distant hybrid combinations occur between genera. Traditionally, morphological and plumage characters have been used to assign the identity of the parental species of a putative hybrid, although recently, nuclear introns also have been used. Here, we demonstrate how high-throughput short-read DNA sequence data can be used to identify the parentage of a putative intergeneric hybrid, in this case between a blue-winged warbler (Vermivora cyanoptera) and a cerulean warbler (Setophaga cerulea). This hybrid had mitochondrial DNA of a cerulean warbler, indicating the maternal parent. For hundreds of single nucleotide polymorphisms within six regions of the nuclear genome that differentiate blue-winged warblers and golden-winged warblers (Vermivora chrysoptera), the hybrid had roughly equal ancestry assignment to blue-winged and cerulean warblers, suggesting a blue-winged warbler as the paternal parent species and demonstrating that this was a first generation (F1) hybrid between these species. Unlike other recently characterized intergeneric warbler hybrids, this individual hybrid learned to song match its maternal parent species, suggesting that it might have been the result of an extra-pair mating and raised in a cerulean warbler nest.

A sexual hybrid and autopolyploids detected in seed from crosses between Neslia paniculata and Camelina sativa (Brassicaceae)

It is important to understand the probability of hybridization and potential for introgression of transgenic crop alleles into wild populations as part of pre-release risk assessment. Here we completed bidirectional crosses between the emerging crop, camelina [Camelina sativa (L.) Crantz] and its weedy relative, ball mustard [Neslia paniculata (L.) Desv.]. Ball mustard is a self-compatible annual that produces hard ball-like seeds similar to canola or mustard seed in size and shape. A total of 1593 crosses were completed and collected with camelina as the maternal parent, while 3253 crosses were successfully collected in the reverse direction. Putatively hybrid seedlings were screened with flow cytometry and species-specific nuclear ribosomal internal transcribed spacer (ITS) markers. Three plants had DNA contents close to expectations for hybrids, but only one of these, formed on camelina, had the expected ITS markers. This hybrid exhibited low fertility, and neither self-pollination nor backcrossing produced viable progeny. The other two plants, formed on ball mustard, had high pollen and seed fertility and were identified as ball mustard neoautotetraploids. Therefore, the hybridization rate between camelina and ball mustard is relatively low at one in 20 000 ovules pollinated when camelina is the maternal parent. However, autotetraploids may form frequently in ball mustard, and tetraploid populations may exist in nature.

Fidelity of Sugarcane Crosses Assessed with SSR Markers

Sugarcane (Saccharum hybrids spp.) is a wind-pollinated species that bears very small yet complete flowers. During crossing, a lack of pollen control (trimming off dehisced florets and hot water treatment) may result in both self-pollination (selfs) and unintentional pollination from stray pollen (off-types). Due to this uncertainty, it is important to estimate the percentage of intentional paternal hybridizations to better understand and possibly improve the crossing process. In this study, six pairs of simple sequence repeats (SSR) primers were used to assess the fidelity of 343 bi-parental crosses made at three U.S. breeding stations in 2002, 2003, 2004, 2005, and 2006. By comparing the SSR fingerprints between the two parents and 20 random progenies, three types of progenies were identified, namely, hybrids, selfs, and off-types. Hybrids had only SSR fingerprints found in either parent; selfs had only SSR fingerprints of the maternal parent; and off-types had SSR fingerprints that were not found in either parent. The fidelity of the 343 crosses fluctuated from 100% hybrids to 100% off-types. Multinomial analysis indicated that the ratio of hybrids from intentional hybridization to selfs and off-types fluctuated by year and location. Selfing rates (−30.5%) and off-type rates (−9.2%) were lower in crosses from which superior progenies were selected and advanced through 10-year field evaluation and testing, but off-type rates were not significant indicating that selfing may reduce advancement rates more than the off-types. Hot water treatment reduced off-types by 27% and selfing by 20%. This research demonstrates the utility of SSR markers for measuring hybridization rates and highlights the importance of pollen control measures through trimming and hot water treatment of sugarcane flowers.

Maternal inheritance of F1 hybrid morphology and colony shape in the coral genus Acropora

Background The coral genus Acropora contains more than 150 species with very high morphological diversity. This high diversity may have been caused by repeated hybridization via mass spawning. However, we have little information whether hybrids are formed in these corals. Identifying morphological differences between hybrids and their parental species would provide an opportunity to find wild hybrids in the field and to understand how colony shapes of Acropora have become highly diversified throughout evolutionary history. In the two morphologically distinctive coral species Acropora florida and A. intermedia in the Indo-Pacific, their gametes show high rates of bi-directional intercrossing in vitro, and thus these two species are ideal species to investigate the morphological traits of the hybrids. Methods We examined morphological characters of F1 hybrids from A. florida to A. intermedia, which were produced from in vitro crossing experiments. To compare morphological differences, we grew juveniles and mature colonies of reciprocal F1 hybrids (FLOint: A. florida eggs × A. intermedia sperm, and INTflo: A. intermedia eggs × A. florida sperm) and of the parental species (purebreds of A. intermedia and A. florida). We analyzed skeletal morphology such as colony size, branch length, and branching number, and compared them with those of a putative F1 hybrid between A. florida and A. intermedia found in the field. We also confirmed the molecular phylogenetic position of F1 hybrids, parental species, and a putative F1 hybrid using the mitochondrial non-coding region. Results Our morphological analysis revealed that branching number of the F1 hybrids was intermediate relative to the parental species. Moreover, the FLOint hybrids were morphologically more closely related to the maternal species A. florida, and the INTflo hybrids were to A. intermedia. Molecular data showed that A. florida and A. intermedia were clearly divided into two clades, and that F1 hybrids grouped in the clade based on their maternal parent. A very similar pattern to the INTflo hybrids was obtained for the putative F1 hybrid in nature. Discussion Our results revealed that F1 hybrids between two Indo-Pacific species A. florida and A. intermedia had intermediate morphology relative to their parent species but reflected the maternal parent more. Similarity to maternal species in hybrids is opposite to the Caribbean Acropora species that had more paternal morphological characters in hybrids. These results further suggest that some genetic factor in eggs is likely to affect determination of colony shape in the Indo-Pacific. At present, we have considered colonies with intermediate morphs between different species to be intra-specific morphological variation, but they may be real F1 hybrids. Indeed, a putative F1 hybrid represented similar morphological and molecular features to the F1 hybrids, and thus it is plausible to be attributed as a “real” F1 hybrid in nature.