Microsporogenesis,reproductive behavior,and fertility in five Pennisetum species

Summary Microsporogenesis, reproductive behavior, pollen fertility and seed set were studied in Pennisetum basedowii Summerhayes and C. E. Hubbard, 2n = 54; P. macrostachyum (Brough.) Trin., 2n = 54; P. macrourum Trin., 2n = 36; P. polystachion (L.) Schult, 2n = 54; and P. squamulatum Fresen 2n = 54. Meiosis was regular in P. basedowii with primarily bivalent pairing. As many as 54 univalents were observed at metaphase I in P. macrostachyum. A high frequency of univalents at metaphase I in P. macrourum resulted in lagging chromosomes and micronuclei at anaphase I and telophase I, respectively. Pennisetum polystachion and P. squamulatum showed frequent multivalent chromosome associations. Studies of megasporogenesis and embryo sac development in P. basedowii showed sexual reproduction. Pennisetum macrostachyum was highly male sterile with predominantly aposporous apomictic embryo sac development. Pennisetum macrourum, P. polystachion, and P. squamulatum had only aposporous embryo sac development. Seed propagated progenies of these latter three species were uniform and matromorphic, confirming the obligate apomixis nature.     

An apomictic polyhaploid obtained from a pearl millet x Pennisetum squamulatum apomictic interspecific hybrid

Summary In a research program to transfer apomixis from Pennisetum squamulatum Fresen to pearl millet, P. americanum L. Leeke, a polyhaploid plant (2n=21) was discovered in the uniform open-pollinated progeny of an apomictic interspecific hybrid (2n = 41) between pearl millet and P. squamulatum. The polyhaploid was shorter, less vigorous and was smaller morphologically than its maternal parent. It probably originated by parthenogenetic development of a reduced gametophyte in the apomictic interspecific hybrid. The most common metaphase I chromosome association in the polyhaploid was 4 bivalents plus 13 univalents. Irregular chromosome distribution, tripolar spindles, bridges and fragments were observed at anaphase I and telophase I. The polyhaploid was male-sterile and partially female- fertile having multiple aposporous embryo sacs in 95% of the ovules. Seed set was 3% when open-pollinated and 33% when pollinated with pearl millet pollen. Low seed set was due to competition among multiple embryos developing in the same ovule. Seventeen progeny from seed produced under open-pollination on the polyhaploid each had 2n=21 chromosomes and were morphologically uniform and identical to the female parent. The expression of obligate apomixis in the polyhaploid conditioned by the P. squamulatum genome between the simplex and duplex condition indicates that apomictic reproduction is possible in nonpolyploid plants.     

Cytogenetics of double cross hybrids between Pennisetum americanum — P. purpureum amphiploids and P. americanum x Pennisetum squamulatum interspecific hybrids

Summary Pearl millet, Pennisetum americanum L. Leeke-napiergrass, Pennisetum purpureum Schum. amphiploids (2n=42) were crossed with pearl millet X Pennisetum squamulatum Fresen. interspecific hybrids (2n=41) to study the potential of germplasm transfer from wild Pennisetum species to pearl millet. These two interspecific hybrids were highly cross-compatible and more than two thousand trispecific progenies were produced from 17 double crosses. All doublecross hybrids were perennial and showed a wide range of morphological variations intermediate to both parents in vegetative and inflorescence characteristics. Some crosses resulted in sublethal progenies. Chromosomes paired mainly as bivalents (¯x15.88) or remained as univalents. At metaphase I, trivalents, quadrivalents, an occasional hexavalent and a high frequency of bivalents indicated some homeology among the genomes of the three species. Delayed separation of bivalents, unequal segregation of multivalents, lagging chromosomes, and chromatin bridges were observed at anaphase I. Although approximately 93% of the double-cross hybrids were male-sterile, pollen stainability in male-fertile plants ranged up to 94%. Seed set ranged from 0 to 37 seed per inflorescence in 71 plants under open-pollinated conditions. Apomictic embryo sac development was observed in double-cross progenies when crosses involved a pearl millet x P. squamulatum apomictic hybrid as pollen parent. These new double-cross hybrids may serve as bridging hybrids to transfer genes controlling apomixis and other plant characteristics from the wild Pennisetum species to pearl millet.     

鹅观草属——人工合成双二倍体(2n=10x=70)的细胞学及育性的研究

本文对缘毛鹅观草(Roegneria pendulina)、鹅观草(R.tsukushiensis var.transiens)及其人工合成杂种F_1、双二倍体进行了细胞学,育性等的分析和研究。结果表明双亲的减数分裂,花粉育性和结实性均正常,杂种F_1的减数分裂不规则且完全不育;当代双二倍体的染色体数目为70,其减数分裂构型为:6.04 Ⅰ+26.21 Ⅱ+1.52 Ⅲ+1.59 Ⅳ+0.02 Ⅴ:第二代双二倍体的染色体数目为70,个别植株为69,减数分裂构型分别为:4.16 Ⅰ+27.33 Ⅱ+0.50 Ⅲ+2.16 Ⅳ和4.79 Ⅰ26.26 Ⅱ+1.13 Ⅲ+2.13 Ⅳ。与期望染色体配对模式相比,双二倍体中二价体出现的频率有明显增大的趋势。在减数分裂AⅠ和AⅡ分别观察到数目不定的落后单价体,大部份的四分体中出现了微核。双二倍体的育性得到了很大程度的恢复,其花粉可染色性及结实率分别为54.4％和64.0％。     

Production and viability of unreduced gametes in triploid interspecific blueberry hybrids

Summary Three triploid (2n=3x=36) blueberry hybrids were obtained by hand-pollinating approximately 7,000 flowers of tetraploid highbush blueberry cultivars (based on Vaccinium corymbosum L.) with pollen from the diploid species V. elliottii Chapm. Meiotic analysis of these triploids revealed trivalents, bivalents and univalents in all metaphase I cells, with lagging chromosomes evident at anaphase I. Pollen of the three triploids was mostly aborted and did not stain with acetocarmine. However, the three triploids did produce from 0.9%–1.3% giant pollen grains that stained with acetocarmine and were present as monads, dyads or triads, rather than the normal tetrads. Pollination of 10,853 flowers of hexaploid V. ashei Reade cultivars with pollen from the triploids produced 266 berries, which averaged fewer than two fully-developed seeds per berry. One triploid clone showed partial female fertility when crossed to hexaploids, self-pollinated, or intercrossed with other triploids. Ploidy levels of the resulting hybrids were determined.Florida Agricultural Experiment Station Journal Series No. 8672     

Meiosis in a triploid hybrid of <Emphasis Type="Italic">Gossypium</Emphasis>: high frequency of secondary bipolar spindles at metaphase II

Studies on meiosis in pollen mother cells (PMCs) of a triploid interspecific hybrid (3x = 39 chromosomes, AAD) between tetraploid Gossypium hirsutum (4n = 2x = 52,AADD) and diploid G. arboreum (2n = 2x = 26,AA) are reported. During meiotic metaphase I, 13 AA bivalents and 13 D univalents are expected in the hybrid. However, only 28% of the PMCs had this expected configuration. The rest of the PMCs had between 8 and 12 bivalents and between 12 and 17 univalents. Univalents lagged at anaphase I, and at metaphase II one or a group of univalents remained scattered in the cytoplasm and failed to assemble at a single metaphase plate. Primary bipolar spindles organized around the bivalents and multivalents. In addition to the primary spindle, several secondary and smaller bipolar spindles organized themselves around individual univalents and groups of univalents. Almost all (97%) of the PMCs showed secondary spindles. Each spindle functioned independently and despite their multiple numbers in a cell, meiosis I proceeded normally, with polyad formation. These observations strongly support the view that in plant meiocytes bilateral kinetochore symmetry is not required for establishing a bipolar spindle and that single unpaired chromosomes can initiate and stabilize the formation of a functional bipolar spindle.     

Cytology of 2N pollen formation and pollen morphology in diploid lotus tenuis (fabaceae)

Two genotypes of Lotus tenuis Waldst & Kit. ex Willd. PI 204882, a diploid (2n = 2x = 12), were identified as producing 2n pollen (maximum = 6%). The objectives of this research were: (1) to determine the mechanism(s) of 2n pollen formation in the L. tenuis genotypes and (2) to morphologically describe n and 2n pollen using light and scanning electron microscopy. Meiotic studies revealed that 2n pollen resulted from tripolar spindles during anaphase II of microsporogenesis. The 2n pollen germinated well, although abnormal pollen tubes were observed. The genetic constitution of 2n pollen resulting from tripolar spindles is equivalent to first division restitution (FDR) of meiosis. Fresh and air-dried pollen samples exhibited differences in size and shape under light and scanning electron microscopy. The size of diploid (2n) pollen was larger than that of haploid (1n) pollen. Normal haploid (1n) pollen was globose-prolate in shape, while diploid (2n) pollen was tetrahedral in shape.     

Factors affecting seedling recruitment in an invasive grass (Pennisetum setaceum) and a native grass (Heteropogon contortus) in the Hawaiian Islands

In the Hawaiian Islands, native Heteropogon contortus (pili grass) is being replaced by alien grasses, one of which is Pennisetum setaceum (fountain grass). Both grasses depend on seeds for population growth. To help understand factors promoting the spread of the alien and decline of the native, we investigated the effects of physical disturbance, nutrient addition, and seed supplementation on seedling recruitment in experimental field plots. In the first year, our field site experienced an unusual drought, and seedling recruitment was greater for H. contortus than for P. setaceum under all treatments. Disturbance increased recruitment of H. contortus seedlings during some sampling periods. Recruitment was not significantly increased by seed additions for either species despite our finding of only 49 and 4 seeds m^–2 in the seed bank for H. contortus and P. setaceum, respectively. In the first year, most P. setaceum seedlings died between monthly surveys. We resurveyed our field plots in a second, wetter year and found the pattern was reversed: recruitment of P. setaceum seedlings was greater than H. contortus seedlings in most treatments. Greenhouse comparisons of seedling survival under three drought regimes (water every 5,7 and 10 days) revealed that H. contortus seedlings tolerate drought better than P. setaceum seedlings. Seedling recruitment for these species in the leeward Hawaiian Islands appears to be primarily dependent on water availability, with the alien having the advantage in wetter years. Once seedlings of the long-lived alien become established, the alien seems capable of maintaining its dominance over H. contortus, even during periods of drought.     

Use of 2n pollen-producing triploid hybrids to introduce tetraploid Mexican wild species germ plasm to cultivated tetraploid potato gene pool

Summary Tetraploid (2n=4x=48) 2EBN Mexican wild species in the series Longipedicellata, which consists of Solanum fendleri, S. hjertingii, S. papita, S. polytrichon, and S. stoloniferum, were crossed with two 2EBN cultivated diploid (2n=2x=24) clones. The resulting triploid hybrids (2n=3x=36) produced 2n pollen (triplandroids) by the mechanism of parallel orientation of anaphase II spindles. The percentage of stainable pollen in 520 triploids ranged between 0 and 23.5%, with a mean of 2.7%. Triploids producing between 13.0 and 23.5% stainable pollen were crossed as staminate parents to the tetraploid cultivars, resulting in abundant pentaploid (2n=5x=60) and near-pentaploid hybrid progeny. Crosses of triploids with lower percentage of stainable pollen as pollen parent to the tetraploid cultivars did not yield fruit, unless rescue pollen from a tetraploid cultivar was added 2 days later. Pentaploid hybrids were selected among selfed tetraploid progenies using morphological and isoxyme markers transmitted from their cultivated diploid parents. These pentaploid hybrids were vigorous and had uniformly sterile pollen. They were female fertile and were crossed with tetraploid cultivars, yielding an average of 19 seeds per fruit. Triplandroids provide the opportunity of transferring 2EBN tetraploid Mexican wild species in the series Longipedicellata germ plasm into the 4EBN cultivated potatoes.Cooperative investigations of the ARS, USDA, and the Washington State University Agricultural Research Center, Prosser, WA 99350, USA. H/LA Paper No. 90-03, College of Agriculture and Home Economies Research Center, Washington State University, Pullman, WA 99164, USA     

RELATIVE SUCCESS OF SELF AND OUTCROSS POLLEN COMPARING MIXED- AND SINGLE-DONOR POLLINATIONS IN AQUILEGIA CAERULEA

Flowers frequently receive both self (S) and outcross (OC) pollen, but S pollen often sires proportionally fewer seeds. Failure of S pollen can reflect evolved mechanisms that promote outcrossing and/or inbreeding depression expressed during seed development. The relative importance of these two processes was investigated in Aquilegia caerulea, a self-compatible perennial herb. In the field I performed single-donor (S or OC) and mixed-donor (S plus OC) pollinations to compare the relative success of both pollen types at various stages from pollen germination to seed maturity. Single-donor S pollinations produced significantly fewer and lighter seeds (x decrease = 12% and 3%, respectively) than OC pollinations. Abortion rates differed by an average of 38% whereas fertilization rates differed by only 5%, indicating that most differences in seed number arose postzygotically. This suggests that inbreeding depression was responsible for most failure of S pollen. One prezygotic effect measured was that 10% fewer S than OC pollen tubes reached ovaries after 42 hr, suggesting S pollen might fertilize proportionately fewer ovules after mixed pollination. Using allozyme markers, I found mixed-donor pollinations produced significantly more and heavier outcrossed than selfed seeds. However, the proportion of selfed seed, fertilized ovules, and aborted seeds for mixed-donor fruits were each predictable from pollen performance in single-donor fruits, suggesting that differential paternity is best explained by inbreeding depression during seed development. Even given these similarities between mixed- and single-donor fruits in the relative performance of S and OC pollen, both individual seed weight and seed set were significantly higher in multiply-sired fruits.     

Reproductive interactions between hybridizing irises: analyses of pollen-tube growth and fertilization success

Pollen-tube growth and seed siring ability were measured in crosses between the Louisiana iris species Iris fulva and Iris hexagona and their F₁ and F₂ hybrids. Flowers of the parental species were pollinated with self, outcross intraspecific, and interspecific pollen. Pollen-tube lengths were similar for all three pollen types in I. fulva, but in I. hexagona interspecific pollen tubes were longer than intraspecific pollen tubes. Pollen-tube lengths also differed for F₁ and F₂ flowers pollinated with I. fulva, I. hexagona, and hybrid pollen. For both hybrid classes I. fulva pollen tubes were the shortest while pollen tubes from I. hexagona and hybrids grew the furthest. Mixtures of genetically marked pollen were used to assess the seed siring ability of intra- and interspecific pollen in the parental species by varying the proportion of each pollen type in a replacement series design. For both species, the observed proportions of hybrid seeds were lower than the expected based on the frequency of each pollen type in the mixtures across all treatments. Flowers of I. fulva produced less than 10% hybrid progeny even when 75% of the pollen applied to stigmas was derived from interspecific flowers. The frequency of hybrid seed formation was somewhat greater in I. hexagona, but was still significantly lower than expected across all mixture treatments. Seed set per fruit remained constant across the mixture treatments for both species, but in I. fulva fruit set decreased with an increase in the proportion of interspecific pollen. The data indicate that both pre- and postfertilization processes contribute to discrimination against hybrid seed formation.     

The use of Endosperm Balance Number and 2n gametes to transfer exotic germplasm in potato

Summary A triploid hybrid (2n=3x=36) between a colchicine-induced 4x(2EBN) Solanum brevidens (a non-tuber-bearing species) and 2x(2EBN) S. chacoense (a tuber-bearing species) was used as a vehicle for germplasm transfer to S. tuberosum Group Tuberosum. The use of 2n gametes from the triploid allowed the unique opportunity for transferring exotic germplasm from Series Etuberosa to Gp. Tuberosum material. The triploid hybrid used had a pollen stainability of less than 0.1%. Observations of microsporogenesis revealed that metaphase I pairing configurations were primarily 12 bivalents and 12 univalents with occasional trivalents. Anaphase I separations were irregular, often with lagging univalents. Meiotic observations and pollen morphology suggest that the stainable pollen produced by the hybrid was 2n=3x=36. A single pentaploid hybrid (2n=5x=60) was produced by the fertilization of a rare 2n egg from the triploid with a normal male gamete from the clone Wis AG 231 (2n=4x=48). Limited crosses to other 1, 2 and 4EBN species and cultivars were unsuccessful. The pentaploid hybrid had a more regular meiosis than the triploid and dramatically improved pollen stainability (37% stainable pollen). Stylar blocks prevented estimates of male fertility in crosses. Female fertility in 47 crosses with nine cultivars averaged 19 seeds per fruit. Although S. brevidens is non-tuber-bearing, and the triploid produced only stolons, the pentaploid hybrid tuberized well under field conditions, despite being very late. Results suggest that the tuberization response is a dosage and/or threshold effect. This approach to the incorporation of 1EBN germplasm indicates the utility of the EBN concept coupled with 2n gametes. Further, it demonstrates a means for the introgression of 1EBN species genes into Gp. Tuberosum material.     

GENOME ANALYSIS OF AGROPYRON REPENS × AGROPYRON CRISTATUM SYNTHETIC HYBRIDS

Hexaploid A. repens, 2n = 42, and diploid A. cristatum, 2n = 14, were hybridized and gave rise to two 28-chromosome reciprocal hybrids. Approximately 1% of hand-emasculated florets of both parent species produced viable hybrid seed following controlled pollination. Early embryo abortion prevented greater hybrid seed set on A. repens, whereas failure of fertilization appeared to be the major cause of poor hybrid seed set on A. cristatum. Reciprocal differences in hybrid vegetative and spike morphology were striking. The A. repens × A. cristatum hybrid was vigorous, highly rhizomatous, and bore abundant spikes whose morphology was intermediate between that of the parent species. A. cristatum × A. repens hybrids were weak, non-rhizomatous with frequently-malformed spikes. Mean chromosome associations of 0.10 I, 20.10 II, and 0.43 IV were observed in 134 metaphase-I cells of A. repens. Subsequent meiotic stages were regular except for occasional laggards and bridges at anaphase I and II. Metaphase-I chromosome associations averaged 0.07 I and 6.97 II in 124 A. cristatum cells. Chromosome pairing in the hybrids was highly variable and averaged 11.45 I, 7.58 II, 0.44 III, and 0.02 IV per cell in 187 cells interpreted. From 5 to 14 laggards appeared in every hybrid cell at anaphase I. Bridges were observed in approximately 25% of the anaphase-I cells. Similar irregularities were observed at anaphase II. Pollen viability was estimated as 3%, and the hybrids failed to set viable seed. On the basis of chromosome pairing in the species itself and in the hybrids, A. repens was designated as a segmental autoallohexaploid with a genome formula of the type A₁A₁A₂A₂BB. Although A. repens and A. cristatum chromosomes paired occasionally, the genomes of the 2 species were essentially non-homologous. Some of the interpretational difficulties of genome analysis were discussed.     

APOMIXIS IN THE POA SECUNDA COMPLEX

Observation of cleared ovules of Poa secunda Presl showed that plants produce aposporous embryo sacs, and emasculation studies showed that they do not set seed without pollen. The ratio of sexual to asexual embryo sacs on a given plant does not correlate with percent stainable pollen, nor does pollen stainability correlate with meiotic regularity or with chromosome number. Percent apomictic ovules, percent pollen stainability, and regularity of meiosis all vary within a given population; they also vary if the same plant is moved from one site to another. Although there is morphological variation in the species, none of the reproductive features correlates with morphology. Pollen from any morph will stimulate seed set in any other morph, and hybrids have been produced in low frequency. Plants will also set seed using their own pollen. In its reproductive biology, P. secunda exhibits many similarities with P. pratensis L.     

Discovery of a synaptic mutant in potato haploids and its usefulness for potato breeding

Summary A synaptic mutant was found in haploids (2n=2x=24) extracted from the Mexican potato variety Atzimba (2n=4x=48). The mutant is inherited as a simple Mendelian recessive, designated sy4. Meiotic abnormalities of the mutant during microsporogenesis include: poor synapsis at pachytene; high frequency of univalents at diakinesis; elongated and curved spindles and univalents being scattered over the spindles at metaphase I and anaphase I; abnormal chromosome distribution at anaphase I; and production of sterile pollen, presumably due to unbalanced chromosome complement. The expression of sy4 in megasporogenesis was also detected. The sy4 mutant is very useful for potato breeding when combined with another meiotic mutant, parallel spindles (ps), because haploids homozygous for sy4 and ps produce fertile 2n pollen which transmit almost intact genotypes of the parents to the progenies. Thus, the meiotic mutants provide a powerful breeding method for maximizing heterozygosity and epistasis. They can also provide a very efficient method of transferring diploid germplasm, which has desired characteristics efficiently combined at the 2x level, to tetraploids. Many haploids have been identified with 2n pollen production by ps alone or by sy4 and ps, vigorous growth and good flowering, and a high level of resistance to late blight. The importance of a further search for meiotic mutants and their use for breeding is discussed.     

RELATIONSHIP OF WOODY PARTHENIUM ARGENTATUM AND HERBACEOUS P. HISPIDUM VAR. AURICULATUM (ASTERACEAE)

Interspecific hybrids between woody Parthenium argentatum Gray (guayule), native to Mexico and Southwest Texas, and herbaceous perennial P. hispidum var. auriculatum (Britton) Rollins, native to the United States, were obtained successfully. The F₁ hybrids were intermediate for most morphological characters with the exception of the short woody stem, yellow pollen color, and the trichome morphology. Chromosome counts revealed the presence of 2n = 36 A-chromosomes in P. argentatum. The same number of A-chromosomes and four B-chromosomes were found in P. hispidum var. auriculatum. Observations of pollen mother cells showed regular meiosis in both parental species. At diakinesis, chiasmata averaged 1.12 and 1.24 per bivalent for P. argentatum and P. hispidum var. auriculatum, respectively. Meiotic behavior of the F₁ hybrids was irregular. F₁ hybrids averaged 4.43 univalents at metaphase I, 1.95 laggards at anaphase I, and 1.62 micronuclei at the tetrad stage. The low pollen stainability (5.1%) in the F₁ hybrids and the limited number of viable BC₁ seeds (4.07%) may be reflections of the irregular meiosis. Although these primary hybrids are partially fertile, they can be used to introduce desirable characteristics of P. hispidum var. auriculatum, such as herbaceous perennial habit, regrowth ability, and cold tolerance into guayule.     

THE EFFECT OF DELPHINIUM NELSONII POLLEN ON SEED SET IN IPOMOPSIS AGGREGATA,A COMPETITOR FOR HUMMINGBIRD POLLINATION

Sympatric plant species can compete for pollination services in several ways. For example, pollinators may move between species and deposit heterospecific pollen on stigmas, which in turn may reduce the efficacy of conspecific pollen. We explored this possibility by determining the effect of Delphinium nelsonii pollen on seed set in Ipomopsis aggregata. These montane herbs are pollinated by hummingbirds, experience heterospecific pollen deposition in nature, and suffer reduced seed set in each other's presence. We hand-pollinated flowers of I. aggregata with either pure conspecific pollen or a mixture of pollen of the two species. Resulting pollen loads on stigmas ranged from 0–865 D. nelsonii grains and from 10–336 I. aggregata grains; mean seed set per flower was 11.3. There was no detectable effect of D. nelsonii pollen load on I. aggregata seed set. It is possible that seed set reductions seen in previous studies of competition for pollination between these species were caused by pollen wastage, pollen layering on the pollinator, or the temporal sequence of pollen arrival at the stigma.     

Panicum shastense (gramineae), a sterile hybrid between P. Pacificum and P. Scribnerianum

Natural and synthetic hybrids are reported betweenPanicum, scribnerianum Nash and two other species,P. pacificum Hitchc. & Chase andP. occidentale Scribn. & Merr. The hybrids are morphologically intermediate between the parents, and those in whichP. pacificum is one parent strongly resemble the rareP. shastense Scribn. & Merr. All the parent plants studied cytologically had regular meiosis and a gametic number ofn = 9 chromosomes. The hybrids usually showed good pairing of chromosomes in meiosis, but irregularities such as univalents and lagging chromosomes were common. In the hybrids generally less than 7% of the pollen grains stained with cotton blue in lactophenol, and the plants set no seed, whereas the parent plants generally had over 90% pollen stainability and a high percentage of seed set. Pollen fromP. shastense, like that of the hybrids, was 3–4% stainable.Panicum shastense is believed to be based on a first generation sterile hybrid and the nameP. xshastense is proposed.     

Chromosome evolution in the Gibasis linearis alliance (Commelinaceae)

Two related species of Gibasis, G. venustula (x = 6) and G. speciosa (x = 5) have been shown to be differentiated by a Robertsonian fusion. Meiotic analysis of the F₁ hybrids has revealed further chromosome differentiation of the parent species, involving several interchanges and inversions. These rearrangements tend to be concentrated in the Robertsonian group, and give rise to unusual meiotic configurations. The sets of the two species are nonetheless highly homologous as shown by high chiasma frequency and as much as 65% stainable pollen in the F₁. Models are proposed suggesting the possible evolutionary pathways of this karyotypic differentiation. The results are discussed in terms of chromosome evolution and its consequences for these species.     

Effects of seed-irradiation on meiosis in M1 plants of Lactuca and related plants

Meiosis was studied in control and seed-irradiated materials of Lactuca sativa, cultivars Tom, Cos and Webbs, L. serriola from the wild, and Cichorium intybus. In meiosis of the M₁ plants from irradiated seed, observations of univalent and multivalent formation, bridge formation at Anaphase I and II, micronucleus formation and persistence, and pollen fertility, were carried out, and compared with controls. Quantitative estimations of bridge formation were made in L. sativa Tom and Cos and in L. serriola, at anaphase I only. It was found that the frequency of anaphase I bridges and reduction of pollen fertility in the M₁ and also M₂ generation, were different for each variety or species; these could be sequenced radiation-resistant to sensitive in the order Tom, Cos and L. serriola.