HYBRIDS AND PHYLOGENETIC SYSTEMATICS II. THE IMPACT OF HYBRIDS ON CLADISTIC ANALYSIS

I examined three aspects of the cladistic treatment of a set of 17 F₁ hybrids of known parental origin: (1) impact of hybrids on consistency index (CI) and number of most parsimonious trees (Trees), (2) placement of hybrids in cladograms, and (3) impact of hybrids on hypotheses of relationship among species. The hybrids were added singly and in randomly selected sets of two to five to a data set composed of Central American species of Aphelandra (including the parents of all hybrids). Compared to analyses with the same number of OTUs all of which were species, the analyses with hybrids yielded results with significantly higher CI. There was no difference in Trees between analyses with hybrids versus species. There was thus no evidence that hybrids would appear to be more problematic for cladistic methods than species. Accordingly, hybrids will not be readily identifiable as taxa that cause marked change in these indices. About % of the hybrids were placed as the cladistically basal members of the lineage that included the most apomorphic parent. Relatively apomorphic hybrids were placed proximate to the most derived parent (ca. 13% of hybrids). Other placements occurred more rarely. The most frequent placements of hybrids thus did not distinguish them from normal intermediate or apomorphic taxa. When analyses with hybrids yielded multiple most parsimonious trees, these were no more different from each other than were the equally parsimonious trees that resulted from analyses with species. Most analyses with one or two hybrids resulted in minor or no change in topology. When hybrids caused topological change, they frequently caused rearrangements of weakly supported portions of the cladogram that did not include their parents. When they disrupted the cladistic placement of their parents, they often caused their parents to change positions, with at least one topology bringing the parental lineages into closer proximity with the hybrid placed between them. Hybrids between parents from the two main lineages of the group caused total cladistic restructuring. In fact, the degree of relationship between a hybrid's parents (measured by both cladistic and patristic distance) was strongly correlated with CI (negatively) and with the degree of disturbance to cladistic relationships (positively). Thus, hybrids between distantly related parents resulted in cladograms with low CI and major topological changes. This study suggests that hybrids are unlikely to cause breakdown of cladistic structure unless they are between distantly related parents. However, these results also indicate that cladistics may not be specially useful in distinguishing hybrids from normal taxa. The applicability of these results to other kinds of hybrids is examined and the likely cladistic treatment of hybrids using other sources of data is discussed.     

HYBRIDS AND PHYLOGENETIC SYSTEMATICS I. PATTERNS OF CHARACTER EXPRESSION IN HYBRIDS AND THEIR IMPLICATIONS FOR CLADISTIC ANALYSIS

Interspecific hybridization is considered common among plants, but the methods of cladistic systematics produce only divergently branching phylogenetic hypotheses and thus cannot give the correct phylogeny if an analysis includes hybrids. Empirical studies of the impact of known hybrids on phylogenetic analysis are lacking, and are necessary to begin to understand the problems that we face if hybrids are often included in cladistic analysis. Examination of the implications of hybrids for cladistics must begin with patterns of character expression in hybrids. This study includes 17 hybrids and their nine parental taxa that are Central American species of Aphelandra (Acanthaceae), analyzed using a set of 50 morphological characters. The hybrids are overwhelmingly intermediate as quantitatively scored for phylogenetic analysis. They express maternal and paternal, and primitive and derived characters in equal frequencies, showing no evidence of predominant inheritance of derived character states as has been assumed by most cladists who have considered hybrids theoretically. Because of their known genetic constitution, hybrids were useful in homology assessment and ordering character states. The parental character set was generally robust, but some changes were made to reflect the special evidence offered by the hybrids. These hybrids suggest that the inclusion of hybrids in phylogenetic analysis will not lead to unresolved cladograms with rampant homoplasy, as has been predicted by other authors. Instead, the patterns of character inheritance in these hybrids lead to the prediction that a hybrid will be placed by phylogenetic analysis as a basal lineage to the clade that includes its most derived parent, with relatively little effect on homoplasy. These predictions will be evaluated by incorporation of the hybrids in phylogenetic analyses, to be reported in a subsequent paper.     

花椰菜与黑芥非对称体细胞杂种的鉴定分析

选用兼具抗黑腐病、黑胫病、根肿病的野生种质黑芥(B.nigra)作为供体,具有良好原生质体培养能力的花椰菜(Boleracea var.botrytis L.)基因型"0307"作为受体,供体叶肉原生质体经不同剂量UV照射处理后,通过PEG介导,与不经任何处理的受体下胚轴原生质体融合,培养获得再生植株.形态学观察显示再生植株的表型多样,呈偏向受体花椰菜类型及中间类型分布:SRAP分子标记特征表明,杂种中受体的遗传信息较为完整,而来自供体的特异扩增带丢失量约为20.0%-97.8%,且不同杂种中存在热点丢失序列;约23.0%的再生植株其染色体数目小于供、受体染色体数之和;采用流式细胞仪对再生株的细胞DNA含量进行分析,结果显示,20%的植株其DNA含量小于供、受体DNA含量总和.对22棵再生植株进行了黑腐病人工接种抗性鉴定,17棵再生植株表现良好抗性,初步证明通过非对称体细胞杂交技术已将外源抗性基因转入花椰菜中.实验表明UV辐射处理,导致再生植株中供体遗传信息的部分丢失或者染色体在不同程度上的消减,获得了花椰菜与黑芥的非对称体细胞杂种,但UV的剂量效应不明显.     

NATURAL AND ARTIFICIAL HYBRIDS OF HELIANTHUS MAXIMILIANI X H. GROSSESERRATUS

Long , Robert W. (Ohio Wesleyan U., Delaware.) Natural and artificial hybrids of Helianthus Maximiliani × H. grosseserratus. Amer. Jour. Bot. 46(10): 687–692. Illus. 1959.—An investigation of the occurrence of natural hybridization in two perennial sunflowers, Helianthus Maximiliani and H. grosseserratus, was begun in 1950. Subsequently, artificial F₁, F₂, and first and second backcross generations were produced. Fertility and vigor were high in all these plants, but F₁ plants appeared to excel the others in these characteristics. Observations in the experimental garden were supplemented by examination of chromosomes in pollen mother cells, comparisons of herbarium collections, and study of wild populations. Evidence pointed to close genetic relationship of the species and to the occurrence of natural hybridization in areas of distributional overlap. In 1957 and 1958, field work in these areas resulted in the scoring of 18 natural populations, 3 of which consisted of both parental species plus putative F₁ hybrids. Two explanations are offered to account for the seeming absence of introgression. The results support the conclusion that natural hybridization leads to the establishment of F₁ hybrids and that introgression does not occur to any significant extent. Although both species display a high degree of interfertility, they are distinct morphologically. For this reason, it is advisable to maintain them as separate species.     

甘蓝型油菜和白菜型油菜种间杂种的小孢子培养

利用分离小孢子培养从甘蓝型油菜（Ｂｒａｓｓｉｃａ ｎａｐｕｓ）和白菜型油菜（Ｂ． ｃａｍ ｐｅｓｔｒｉｓ）种间杂种中获得了胚和再生植株。所用的培养程序是,将甘蓝型油菜和白菜型油菜杂种小孢子在蔗糖浓度为１７％ 、ＢＡ 为０．１ ｍ ｇ／Ｌ的液体ＮＬＮ 培养基中３２ ℃下暗培养４８ ｈ,再转入蔗糖浓度为１０％ 的ＮＬＮ 培养液中２５ ℃下暗培养３ 周。不同杂种间小孢子胚胎发生能力存在差异,其中ＵＭ９２１（白菜型油菜）×９１１１８６（甘蓝型油菜）正反交杂种的胚产量显著高于供试的其它组合。供体植株种植在１０ ℃／５ ℃（昼／夜）条件下能显著改善杂种小孢子胚产量和质量。杂种小孢子胚产量和杂种植株每荚种子数存在极显著正相关,但杂种植株的花粉育性和胚产量间相关不显著。大多数甘蓝型油菜和白菜型油菜杂种小孢子胚衍生植株为非整倍体,２２．８％ 的植株起源于具亲本染色体数的小孢子,几乎全部为ｎ＝ １９ 的类型。讨论了影响种间杂种小孢子胚胎发生的因素以及种间杂种小孢子培养技术的可能用途     

CHROMATOGRAPHIC COMPARISON OF DICENTRA SPECIES AND HYBRIDS

Ten species of Dicentra were examined by two-dimensional descending paper chromatography. The flavonoid components, including anthocyanins, formed patterns which were specific to a species or a group of closely related species, and these confirmed certain natural relationships within the genus. Some of the components were hydrolyzed and close structural relationships among them were revealed. They were further characterized by spray reactions and R_F measurements in a variety of solvent systems. The inheritance of the components, especially of the anthocyanins, was studied in the hybrids. All parental substances appeared in some hybrids, but in others some parental components were missing. “Hybrid substances” which had not been present in either parent were found in certain hybrids. This is thought to represent either reconstruction of ancestral biosynthetic pathways, probably through genic complementation, or extension of existing synthetic routes through some type of interaction between parental genomes.     

CHROMATOGRAPHIC COMPARISON OF TRAGOPOGON SPECIES AND HYBRIDS

Approximately 1,700 plants representing five species of Tragopogon (Compositae) and their F₁ and F₂ hybrids were analyzed by two-dimensional descending paper chromatography. Each species, or population within a species, was chromatographically distinct. Often, however, the differences were more quantitative than qualitative. The chromatographic data generally supported the species relationships which had been determined from previous morphologic, hybridization, and fertility studies. Inheritance of the flavonoid compounds was usually additive in the F₁'s. Segregation and recombination of the genes controlling the synthesis of these compounds sometimes approximated 3:1 or 9:7 ratios in the F₂'s. Occasionally parental compounds were missing from some of the hybrids. “Hybrid” compounds which had not been found in either parent were absent from the F₁ but did occur in several F₂ populations. Two linkage groups were present. The first contains genes controlling the synthesis of three compounds and the second, four compounds.     

INTERSPECIFIC HYBRIDS OF DROSOPHILA HETERONEURA AND D. SILVESTRIS I. COURTSHIP SUCCESS

Drosophila heteroneura and D. silvestris are well-defined, sympatric species of the planitibia subgroup of Hawaiian Drosophila. D. silvestris can be subdivided into two allopatric morphotypes that differ in the number of bristle rows on the front tibia (two rows versus three rows). We measured courtship success of intraspecific and interspecific hybrids as the proportion of females inseminated during a two-week period with a single sib male. Proportions were arcsin-transformed so that the values were asymptotically normal in distribution, and tests of homogeneity and of mean differences were performed. Of key importance is the discovery of genetic variation for the proportion of inseminated females within both D. heteroneura and D. silvestris. The interspecific crosses and the D. silvestris intraspecific crosses also provide evidence for a coadapted gene complex with some dominance or heterosis. This coadapted gene complex correlates with the morphotypes of these flies, rather than with the D. heteroneura/D. silvestris contrasts per se. This observation stresses the importance of recognizing both behavioral and morphological components of the mate-recognition system. The incompatible coadaptation that separates the two-row from the three-row forms also supports recent molecular studies which indicate that the three-row form split from the two-row form prior to the split between D. heteroneura and two-row D. silvestris. The observations of intraspecific variability and coadaptation support the predictions of a genetic-transilience model which explains the origin of a new mate-recognition system in terms of sexual selection in the context of a founder-flush event.     

NATURAL HYBRIDS BETWEEN ORYZOPSIS AND STIPA. I. ORYZOPSIS HYMENOIDES × STIPA SPECIOSA

Johnson , B. Lennart . (U. California, Los Angeles.) Natural hybrids between Oryzopsis and Stipa. I. Oryzopsis hymenoides × S. speciosa. Amer. Jour. Bot. 47(9): 736–742. Illus. 1960.—Spontaneous, sterile hybrids between Oryzopsis hymenoides and Stipa speciosa heretofore have been referred to O. bloomeri, which species in an earlier paper was shown to consist of a series of hybrids between O. hymenoides and various species of Stipa. On many morphological characters (O. hymenoides × speciosa) is intermediate between its parents and similar to the other hybrids formerly classed as O. bloomeri. It is distinct from the other hybrids on characters which reflect S. speciosa, except with respect to indument of the awn which is recessive. Somatic chromosome counts are: O. enoides, 48; S. speciosa, 64; hybrid, 56. Both parents exhibit regular bivalents at meiosis. The hybrid shows only occasional bivalents and mostly asynchronous splitting or random distribution of univalents at metaphase I. Pollen abortion is complete. No evidence of natural amphiploidy or introgression was found.