Pollen competition and environmental effects on hybridization dynamics between Phlox drummondii and Phlox cuspidata

Pollen competition between species strongly influences hybridization dynamics in plants. By performing single- and mixed-donor pollinations, we show that soil Ca alters the outcome of interspecific pollen competition in the annual Phlox hybrid system of Phlox cuspidata and P. drummondii. In the absence of interspecific pollen competition, heterospecific pollen siring success of both species was influenced most strongly by the maternal growth environment, such that hybridization was facilitated when heterospecific pollen was deposited on stigmas of maternal plants growing in high Ca soils. When heterospecific pollen was forced to compete against conspecific pollen, however, the maternal growth environment did not influence hybridization, but the environmental origin of heterospecific pollen did, and this effect depended on the maternal species. Pollen of P. drummondii was more effective at outcompeting P. cuspidata pollen and preventing hybridization in P. drummondii dams when P. cuspidata pollen was derived from low Ca. Pollen competition within pistils of P. cuspidata was unaffected by pollen Ca environment. In situations in which P. cuspidata grows in lower soil Ca than P. drummondii, as has been documented in one population, these results suggest that the competitive ability of heterospecific pollen will be diminished by environmental effects of soil Ca. Thus, the environment in which pollen develops can influence interspecific pollen competition and hybridization frequency.     

Mating system and the hybridization between self-compatible Phlox cuspidata and self-incompatible Phlox drummondii

Mating system can impact the frequency of hybridization and therefore the maintenance of species diversity. I evaluate the effects of weak self-incompatibility (SI) in Phlox cuspidata and SI in Phlox drummondii on mating success within species and on hybridization dynamics between species under controlled conditions. The effects of SI on hybridization frequency were assessed by manipulating the relatedness of conspecific pollen and the relative timing of pollen deposition in mixed-donor interspecific pollinations. Selfing as opposed to outcrossing increased hybridization by 16% in P. cuspidata maternal plants and by 48% in P. drummondii maternal plants because self pollen did not compete as well against heterospecific pollen. The relative timing of conspecific versus heterospecific pollen deposition also impacted hybridization. In self-compatible P. cuspidata, the deposition of self pollen 5 h earlier than heterospecific pollen decreased hybridization by 28%. In self-incompatible P. drummondii, a 5 h delay in the deposition of compatible conspecific pollen increased hybridization by 32%. In this hybrid system, early self-pollination can decrease hybridization (but increase inbreeding) by P. cuspidata maternal plants, and SI may increase hybridization by P. drummondii maternal plants.     

HYBRIDIZATION BETWEEN ANNUAL SPECIES OF PHLOX: POPULATION STRUCTURE

Phlox cuspidata (n = 7) and P. drummondii subsp. drummondii (n = 7) are closely related annuals which are indigenous to eastern and central Texas. The species typically occupy different ecological niches but may form contiguous or confluent populations in disturbed habitats and hybridize therein. On the basis of correlative interpretations of exomorphic, chromatographic and fertility information, hybridizing populations can be segregated into three distinct classes: (1) highly fertile plants with the morphological and phenolic attributes of P. drummondii; (2) highly fertile plants with the morphological and chemical attributes of P. cuspidata; (3) sterile plants with manifestly intermediate morphology and complementary chromatographic patterns. These data strongly suggest that hybridizing populations of P. drummondii and P. cuspidata are tritypic, being composed of “pure” or essentially “pure” parental species and a group of plants which has all of the attributes characteristic of an F₁ hybrid.     

QUANTITATIVE VARIATION IN PHLOX: COMPARISON OF SELFING AND OUTCROSSING SPECIES

Variation of 20 quantitative characters was examined within and among 10 populations of the predominantly outcrossing Phlox drummondii and 4 populations of the predominantly selfing P. cuspidata grown in a greenhouse. Multivariate analysis of variance, considering all characters simultaneously, indicated that there were significant differences among populations in both species while analysis of individual characters demonstrated that there were significant population differences for 19 characters in P. drummondii and 13 characters in P. cuspidata. On average, 16% of the total phenotypic variation in P. drummondii occurred among populations compared to less than 4% of the total variation in P. cuspidata. In addition, P. drummondii exhibited significant differences among families within populations more frequently than P. cuspidata. Most observed variation in both species occurred within families where environmental and genetic sources of variation could not be partitioned. There was a trend for P. drummondii to have higher heritabilities than P. cuspidata for most characters even when assumptions about breeding systems were relaxed. Thus, the outbreeding species exhibited larger genetic differences among populations and among families within populations than the selfing species in the greenhouse environment. These data suggest that P. drummondii has the greater evolutionary potential of the two species and are consistent with the hypothesis that differences in population structure result from differences in the breeding systems of the two species.     

REPRODUCTIVE CHARACTER DISPLACEMENT IN PHLOX

Plants of the red- and pink-flowered P. drummondii were introduced into a natural population of P. cuspidata. Their seeds were progeny-tested, and the percentage hybrid seed determined for each variant. Thirty-eight percent of the seed from the pink variant were hybrid compared to 13% of the red. The difference in hybrid production is best explained by corolla color. The self- and cross-compatibility of nine sympatric and nine allopatric populations were studied in the greenhouse. On the average, sympatric populations are more self-compatible than allopatric populations. Pollen germination is 13.7% vs. 7.3%, the difference being statistically significant. The self-compatibility differential was accompanied by a self seedset differential. On the average, sympatric populations are more cross-compatible with P. cuspidata than are allopatric populations. The germination of P. cuspidata pollen on sympatric P. drummondii stigmas was 13.3% vs. 9.8% on allopatric stigmas. However, the difference is not statistically significant. The presence of P. cuspidata has promoted reproductive character divergence in P. drummondii. The shift in corolla color and the increase in self-compatibility reduce the potential for gametic wastage and interspecific hybridization.     

Unidirectional hybridization and reproductive barriers between two heterostylous primrose species in north-west Yunnan,China

Background and Aims

Heteromorphy in flowers has a profound effect on breeding patterns within a species, but little is known about how it affects reproductive barriers between species. The heterostylous genus Primula is very diverse in the Himalaya region, but hybrids there have been little researched. This study examines in detail a natural hybrid zone between P. beesiana and P. bulleyana.

Methods

Chloroplast sequencing, AFLP (amplified fragment length polymorphism) markers and morphological comparisons were employed to characterize putative hybrids in the field, using synthetic F₁s from hand pollination as controls. Pollinator visits to parent species and hybrids were observed in the field. Hand pollinations were conducted to compare pollen tube growth, seed production and seed viability for crosses involving different morphs, species and directions of crossing.

Key Results

Molecular data revealed all hybrid derivatives examined to be backcrosses of first or later generations towards P. bulleyana: all had the chloroplast DNA (cpDNA) of this species. Some individuals had morphological traits suggesting they were hybrids, but they were genetically similar to P. bulleyana; they might have been advanced generation backcrosses. Viable F₁s could not be produced with P. bulleyana pollen on P. beesiana females, irrespective of the flower morphs used. Within-morph crosses for each species had very low (<10 %) seed viability, whereas crosses between pin P. bulleyana (female) and pin P. beesiana had a higher seed viability of 30 %. Thus genetic incompatibility mechanisms back up mechanical barriers to within-morph crosses in each species, but are not the same between the two species. The two species share their main pollinators, and pollinators were observed to fly between P. bulleyana and hybrids, suggesting that pollinator behaviour may not be an important isolating factor.

Conclusions

Hybridization is strongly asymmetric, with P. bulleyana the only possible mother and all detected hybrids being backcrosses in this direction. Partial ecological isolation and inhibition of heterospecific pollen, and possibly complete barriers to F₁ formation on P. beesiana, may be enough to make F₁ formation very rare in these species. Therefore, with no F₁ detected, this hybrid zone may have a finite life span as successive generations become more similar to P. bulleyana.     

Intergeneric hybridization between Diplotaxis siifolia,a wild species and crop brassicas

Summary Attempts were made to obtain intergeneric hybrids between Diplotaxis siifolia, a wild species, and cultivars of Brassica (B. campestris, B. juncea, and B. napus). The crosses showed unilateral incompatibility. When the wild species was used as female parent, pollen germination and pollen tube growth were normal, but hybrid seeds aborted due to post-fertilization barriers. Reciprocal crosses (cultivars as female parent) showed strong pre-fertilization barriers; although pollen grains showed germination, pollen tubes failed to enter the stigma. Hybrids were realized in two of the crosses, D. siifolia x B. juncea and D. siifolia x B. napus, through ovary culture. The hybrids were multiplied in vitro by multiplication of axillary shoots, or somatic embryogenesis. Detailed studies were carried out on the hybrid D. siifolia x B. juncea. F₁ hybrids had shrivelled anthers and were pollen sterile. Amphiploids of this hybrid showed 60% pollen fertility and produced seeds upon self-pollination as well as backcross pollination with the pollen of B. juncea.     

Seedling establishment ofCastanopsis cuspidata var.sieboldii andPersea thunbergii on lava and scoria of the 1962 eruption on Miyake-jima Island,the Izu Islands

Seedling establishment ofCastanopsis cuspidata var.sieboldii Nakai andPersea thunbergii Kosterm. (Machilus thunbergii Sieb. et Zucc.) in early succession was studied on the lava and the scoria of the 1962 eruption on Miyake-jima Island of the Izu Islands. While tall individuals (> 1 m) ofP. thunbergii were found on both the lava and the scoria, those ofC. cuspidata (> 1 m) were found only on the lava. The height growth rate ofC. cuspidata was very low on the scoria, and there was no individual ofC. cuspidata taller than 0.4 m on the scoria. The proportion of yellow leaved individuals was much higher (37.1%) inC. cuspidata than inP. thunbergii (4.0%) on the scoria, while it was similar between the two species on the lava.Persea thunbergii seemed to be able to establish on both the lava and the scoria, butC. cuspidata established only on the lava. The differences in growth rate and vigor between the two species on the lava and the scoria in early succession may influence later stages of succession.     

The association between pollen size and Renner complex in Oenothera villaricae and O. picensis ssp. picensis and their hybrids: evidence for preanthesis pollen competition

In both Oenothera villaricae Dietrich and O. picensis ssp. picensis, chromosomes are transmitted as two Renner complexes. Reciprocal combinations of the Renner complexes produce eight different F₁ hybrids, but only seven are viable. Each species, and each F₁ hybrid, produces three sizes of pollen, approximately 50% small sterile grains, 15% medium-sized viable grains and 35% large viable grains. Medium- and large-sized grains were separated manually and subjected to random amplified polymorphic DNAs (RAPDs) analysis. A pattern of RAPD amplifications was obtained which indicates that, for each species and F₁ hybrid, one specific Renner complex characterizes the medium- and another the large-sized viable pollen. The results indicate that pollen size is determined in part by the pollen genotype and in part by the genotype of the other pollen grains developing within the same anther.     

Distribution of sawfishes (Pristidae) in the Queensland Gulf of Carpentaria,Australia, with notes on sawfish ecology

Synopsis Pristis microdon, P. zijsron, P. clavata and Anoxypristis cuspidata are distributed throughout the Queensland section of the Gulf of Carpentaria, Australia. In a survey of the four species, Anoxypristis cuspidata was the most abundant and was recorded in both the inshore and offshore set net fisheries. The size distribution and catch locations of A. cuspidata suggest that the inshore area to a depth of 10 m may be the preferred habitat for juveniles of this species, while adults primarily occur offshore. Pristis microdon, P. zijsron and P. clavata were recorded only in the inshore fishery with catches dominated by immature animals. Pristis microdon was caught in the inshore fishery late in the monsoonal wet season (February to April) and inhabited both freshwater and estuarine environments. Pristis zijsron occurred only on the sand and mud flats outside river mouths whilst P. clavata inhabited both the sand and mud flats and upstream estuarine habitats. Observations on reproductive staging and the capture of neonate specimens suggest that in all four pristids, pupping occurred through the wet season until the beginning of the dry season in May. A seasonal set net closure for the barramundi, Lates calcarifer and shark fisheries, which has been in place since 1980 in Queensland Gulf waters, therefore offers a measure of protection to breeding female sawfish and their offspring.     

Fertile,intermediate hybrids between Phaseolus vulgaris and P. acutifolius from congruity backcrossing

Summary Recurrent or introgressive backcrossing of Phaseolus vulgaris — P. acutifolius hybrids with either P. vulgaris or P. acutifolius increased fertility, as measured by pollen stainability and seed per pod from non-manipulated flowers, while invariably resulting in loss of traits from the non-recurrent parent. When hybrids were backcrossed with each of the parent species in alternate generations (congruity backcrossing), fertility decreased in early generations but gradually increased in later generations. By the fourth or fifth generation, congruity-backcross hybrids produced numbers of seeds per pod from non-manipulated flowers comparable to those of parent species, although the percent of stainable pollen tended to be lower. Congruity-backcross hybrids were intermediate to parent species when pedigrees contained equal representation of parents but favored the majority parent when pedigrees were unbalanced. Individuals exhibiting symptoms of developmental incongruity, such as abnormal leaflet number, foliar variagation, or unusual growth patterns, occurred in each generation. However, completely new characteristics appeared after the second backcross generation, presumably due to recombination. Backcrossing to both parent species in alternate generation reversed incongruity, allowing selection for fertility without loss of traits from one of the parent species.Scientific Journal Series Paper Number 15, 671 of the Minnesota Agricultural Experimental Station     

Second generation experimental hybridizations in theEchinocereus ×lloydii complex (Cactaceae), and further documentation of dioecy inE. coccineus

Artificial second generation and backcross hybrids involvingEchinocereus dasyacanthus (Texas rainbow cactus),E. coccineus (a species of claret-cup cactus), andE. ×lloydii (Lloyd's hedgehog cactus) were examined for morphological and reproductive characteristics. The hybrids grew from seed to flower in about four years, and they exhibited high fertility as measured by pollen stainability, crossability, and fruit-set. A rare breeding system inCactaceae, dioecy, was further examined inE. coccineus, using field and controlled pollination tests. Functional dioecy was documented forE. coccineus in Trans-Pecos Texas. All results suggested thatE. ×lloydii in eastern Pecos County, Texas, is a complex and dynamic population probably comprising first and later generation hybrids, including backcross hybrids. The experimental hybridizations also tended to support the occurrence of introgression into both putative parental species.     

Hybrids and backcross progenies between wheat (Triticum aestivum L.) and apomictic Australian wheatgrass [Elymus rectisetus (Nees in Lehm.) A. Löve & Connor]: karyotypic and genomic analyses

Wheat (Triticum aestivum L., 2n = 6x = 42, AABBDD) florets were emasculated and pollinated using two apomictic wheatgrass [Elymus rectisetus (Nees in Lehm.) A. Love & Connor, 2n = 6x = 42, SSYYWW] accessions, one of which produces 2n pollen. A 2n = 42 (B_II) hybrid and four 2n = 63 (B _III) hybrids were obtained. The spike morphology of the B _II hybrid was intermediate to that of its parents. The pollen mother cells (PMCs) of this hybrid contained on average 38.361 and 1.62 II, which was consistent with its disparate genome composition (ABDSYW). Its pollen failed to stain and no BC₁ progeny was obtained. The B _III hybrids (reduced egg fertilized with unreduced sperm) were grasslike and had a full complement of E. rectisetus chromosomes, the synapsis of which was slightly impaired by wheat haplome and/or cytoplasm. Their PMCs contained on average 16.30 II, 25.72 I, and 1.54 multivalents (III plus IV). Pollen stainability in these hybrids was low (<1%), and when they were used as females, one 54- and 60-chromosome BC₁ were obtained. A mean of 13.25 II was observed in PMCs of the 54-chromosome BC₁ and pollen stainability was 10%. Pollen stainability in the 60-chromosome BC₁ was only 5%. The use of 2n-pollen-producing E. rectisetus accession accelerated hybrid and BC₁ formation and may accelerate the ultimate transfer of apomixis to wheat.     

The interspecific hybrid Petunia parodii × P. inflata and its relevance to somatic hybridization in the genus Petunia

Summary Attempts at the reciprocal cross between Petunia parodii and P. inflata using standard emasculation and pollination techniques failed. Limited pollen tube growth down the style in reciprocal crosses led to reproductive isolation between the self-compatible P. parodii and self-incompatible P. inflata. The interspecific hybrid was successfully produced by bud-pollination of P. parodii with P. inflata as the male parent in 22 percent of attempts, but not in the opposite direction. In vitro pollination of P. parodii ovaries with P. inflata pollen also produced hybrids. The small size of the ovary made it technically impossible to use P. inflata as the female parent for in vitro pollination. The interspecific hybrids were intermediate, as compared to the two parents, for six of the seven plant and flower characters measured. Furthermore, the hybrids had high pollen fertility, set abundant seed upon self-pollination, and readily inter-crossed with the parental species. The results are consistent with a high degree of chromosomal homology in the parental species and with minor genetic divergency leading to reproductive isolation that is pre-zygotic in nature. Overcoming the barriers to cross-incompatibility by practical techniques resulted in fertile interspecific hybrids that segregated for parental characters. The potential value of employing the parental species in somatic hybridization experiments is discussed.Michigan Agricultural Experiment Station Journal Article No. 8404     

Hoarding behaviors of two wood mouse species: Different preference for acorns of two Fagaceae species

Two species of wood mouse, Apodemus argenteus and A. speciosus, were observed consuming and hoarding acorns of Quercus serrata and Castanopsis cuspidata. When each species of acorn was supplied individually, both species of mice used each species of acorn for eating and hoarding. When both species of acorn were supplied, A. argenteus consumed or hoarded only C. cuspidata, whereas A. speciosus tended to eat C. cuspidata acorns at the feeding site, and disperse or hoard Q. serrata acorns. Apodemus speciosus is unlikely to disperse C. cuspidata acorns (their utilization was biased towards consumption) when Q. serrata acorns are also available. Apodemus argenteus will make almost no contribution to the dispersal of Q. serrata when the two acorn species coexist.     

Inheritance of rapeseed (Brassica napus)-specific RAPD markers and a transgene in the cross B. juncea x (B. juncea x B. napus)

We have examined the inheritance of 20 rapeseed (Brassica napus)-specific RAPD (randomly amplified polymorphic DNA) markers from transgenic, herbicide-tolerant rapeseed in 54 plants of the BC₁ generation from the cross B. junceax(B. junceaxB. napus). Hybridization between B. juncea and B. napus, with B. juncea as the female parent, was successful both in controlled crosses and spontaneously in the field. The controlled backcrossing of selected hybrids to B. juncea, again with B. juncea as the female parent, also resulted in many seeds. The BC₁ plants contained from 0 to 20 of the rapeseed RAPD markers, and the frequency of inheritance of individual RAPD markers ranged from 19% to 93%. The transgene was found in 52% of the plants analyzed. Five synteny groups of RAPD markers were identified. In the hybrids pollen fertility was 0–28%. The hybrids with the highest pollen fertility were selected as male parents for backcrossing, and pollen fertility in the BC₁ plants was improved (24–90%) compared to that of the hybrids.     

Investigation and documentation of hybridization betweenParkinsonia aculeata andCercidium praecox (Leguminosae:Caesalpinioideae)

Morphometric, cytogenetic, geographical and ecological evidence for hybridization betweenParkinsonia aculeata andCercidium praecox is presented. Morphometric investigation using the character count procedure and cytogenetic observations confirm hybrid status. All diagnostic morphometric characters were intermediate in the hybrid. Both parents (2n = 28) show regular tetrad formation and pollen fertility greater than 94%. Hybrids have a chromosome number of 2n = 28 or 2n = 30, and display meiotic abnormalities including lagging chromosomes and micronucleus formation; less than 21% of hybrid pollen was fertile. Ecological and geographical information suggests that hybridization is occurring at increasing frequency due to the expanding range ofP. aculeata associated with cultivation as an ornamental, coupled with ecological disturbance and weediness, and the cultivation ofC. praecox and hybrids as fodder, ornamental and shade trees. Hybrid fertility and phenological observations, in conjunction with F-weighted principal component analysis, suggest that the progeny of F₁ hybrids are established. The hybrid is formally described asP. ×carterae.     

In-situ immunofluorescent localization of phosphoenolpyruvate and ribulose 1,5-bisphosphate carboxylases in leaves of C3, C4, and C3−C4 intermediatePanicum species

The in-situ inter- and intracellular localization patterns of phosphoenolpyruvate (PEP) and ribulose 1,5-bisphosphate (RuBP) carboxylases in green leaves of severalPanicum species were investigated using an indirect immunofluorescence technique. Four species were examined and compared:P. miliaceum (C₄),P. bisulcatum (C₃), andP. decipiens andP. milioides (C₃–C₄ intermediates which have Kranz-like leaf anatomy and reduced photorespiration). In the C₄ Panicum, PEP carboxylase was located in the cytosol of the mesophyll cells and RuBP carboxylase was restricted to the bundle-sheath chloroplasts. In contrast, in the C₃ Panicum species, PEP carboxylase was found throughout the leaf chlorenchyma, in both the cytosol and chloroplasts, and RuBP carboxylase was located in the chloroplasts. For the C₃–C₄ intermediate plants, the patterns depended on the species examined. ForP. decipiens, the in-situ localization of both carboxylases was similar to that described forP. bisulcatum and other C₃ plants. However, inP. milioides, PEP carboxylase was found exclusively in the cytosol of the mesophyll cells, as inP. miliaceum and other C₄ species, whereas RuBP carboxylase was distributed in both the mesophyll and bundle-sheath chloroplasts.Abbreviations PEP phosphoenolpyruvate - RuBP ribulose 1,5-bisphosphate     

Potato germ plasm enhancement with disomic tetraploid Solanum acaule. II. Assessment of breeding value of tetraploid F1 hybrids between tetrasomic tetraploid S. tuberosum and S. acaule

The breeding value of tetraploid F₁ hybrids between tetrasomic tetraploid S. tuberosum and the disomic tetraploid wild species S. acaule was examined. The F₁ hybrids showed a tuber yield and appearance comparable to those of their cultivated parent, indicating a potential as acceptable breeding stocks despite the 50% contribution to their pedigree from wild S. acaule. The cytological behavior of the tetraploid F₁ hybrids was examined to determine the probability of recombination for the introgression of S. acaule genes. The majority of the meiotic configurations at metaphase I was bivalents and univalents with mean frequencies of 17.6 and 9.9, respectively. Further, a low frequency of trivalents and quadrivalents was observed. An acceptable low level of meiotic irregularities were observed at the later stages of microsporogenesis, and a reasonable level of pollen stainability was obtained. Therefore, these hybrids could likely be employed for further introgression. From the cytological observations, the following speculations were drawn: (1) some genomic differentiation exists between the S. acaule genomes, (2) at least one of the S. acaule genomes may be homoeologous to the S. tuberosum genomes, (3) intergenomic recombination would likely occur due to the nature of the genomic constitution of the hybrids, and (4) the nature of sesquiploidy of the hybrids may facilitate efficient introgression and establishment of unique aneuploid and euploid recombinant genetic stocks.