Ancestral Homologs Research Articles

Larval Shell Muscles in the Abalone Haliotis kamtschatkana.

I used light and electron microscopy to investigate shell-attached muscles in larvae of Haliotis kamtschatkana Jonas, 1845, because an early description of these muscles in H. tuberculata by Crofts (1937, 1955) has featured prominently in theories about gastropod evolution. Larval shell muscles in H. kamtschatkana can be grouped into two categories. The first category consists of the larval retractor muscle (LRM) and the accessory larval retractor muscle (ACC); these are striated muscles in which myofilaments begin differentiating before the head and foot rotate relative to the protoconch (this rotation is known as ontogenetic torsion). Collectively, these muscles ultimately insert on tissues within the larval head and mantle, but the ACC and mantle fibers of the LRM degenerate as metamorphic competence is achieved. The second category consists of two nonstriated pedal muscles that differentiate after cephalopodial rotation. The left pedal muscle is anchored on the back of the protoconch, to the left of the shell-attachment plaque for the LRM. It projects into the foot primarily, but also gives rise to muscle slips extending into the mantle fold. The right pedal muscle is anchored on a calcareous septum secreted along the visceral rim of the protoconch. The new data force a reconsideration of the ancestral homologues of larval shell muscles in abalone, because Crofts may have misidentified the accessory larval retractor muscle as a precursor of one of the later pedal muscles.

The hinge region of chicken annexin I contains no site for tyrosine phosphorylation

Annexin I (AnxI) is a calcium-dependent membrane binding protein which has been implicated in various physiological activities. The region of the chicken anxI cDNA encoding the first 130 amino terminal residues was cloned by reverse transcription PCR in order to determine the relationship of its variable ammo-terminal regulatory region with other known annexins. This nucleotide sequence shows 86% identity with pigeon Anxl isoforms, and 57% with its human homolog. The protein encoded by the chicken anxI cDNA lacks the canonical epidermal growth factor receptor/kinase phosphorylation site, which is present in Anxl of other species. In contrast, the putative protein kinase C phosphorylation site of the amino-tenninus is present in the chicken AnxI. Whereas the pigeon genome contains two anxI genes, genomic Southern analysis shows that in the chicken Anxl is encoded by only a single gene. These data suggest that AnxI has undergone significant sequence variation in the avians, and clarifies the relationships of the avian anxI genes with their ancestral homologs.

Molecular cloning of orangutan and gibbon MHC class I cDNA. The HLA-A and -B loci diverged over 30 million years ago.

To investigate whether the classical HLA MHC class I loci have been preserved during evolution of the primates, we have cloned, sequenced, and expressed eight MHC class I cDNA from orangutan and gibbon lymphocytes. Both the HLA-A and -B loci are present in both of these species. In fact, lymphocytes from the orangutan expressed three HLA-B-related gene products, suggesting that the ancestral homologue of the HLA-B locus had undergone a duplication in this species. Interestingly, several amino acid motifs thought to be important in the Ag-presenting function of MHC class I molecules were preserved in the Ag-recognition sites of the orangutan and gibbon MHC class I molecules. Finally, these findings suggest that the recombination event between the HLA-A and -E loci occurred over 38 million years ago. These data indicate that the HLA-A and -B loci are extremely stable and that recombination between them is rare. Furthermore, the data presented here argue against the role of concerted evolution in the evolution of primate MHC class I molecules.

Evolution of the MHC class I genes of a New World primate from ancestral homologues of human non-classical genes

The products of the classical human major histocompatibility complex (MHC) class I genes (HLA-A, -B, -C) are highly polymorphic molecules that bind peptides and present them to T lymphocytes. The non-polymorphic, non-classical MHC class I gene products (HLA-E, -F, -G) are not restricting elements for the majority of T lymphocytes. The evolutionary relationship of the non-classical and classical MHC class I genes is unclear. Here we present the cloning and sequencing of the MHC class I genes of a New World primate, the cotton-top tamarin (Saguinus oedipus). The expressed MHC class I genes of this species are more closely related to the human non-classical HLA-G gene than they are to genes of the human classical HLA-A, -B, and -C loci. These observations imply that classical and non-classical genes do not necessarily constitute mutually exclusive groups over evolutionary time.

Transcription of the Spec 1-like gene of Lytechinus is selectively inhibited in response to disruption of the extracellular matrix.

The influence of the extracellular matrix (ECM) on differential gene expression during sea urchin development was explored using cell-type-specific cDNA probes. The ECM of three species of sea urchin, Strongylocentrotus purpuratus, Lytechinus variegatus and Lytechinus pictus, was disrupted with the lathrytic agent beta-aminopropionitrile (BAPN), which inhibits collagen deposition in the ECM and arrests gastrulation (Wessel & McClay, Devl Biol. 121: 149, 1987). The levels of several mRNAs (Spec 1, Spec 2, CyIIa actin, CyIIIa actin and collagen in S. purpuratus, and metallothionine, ubiquitin and LpS3 in L. pictus and L. variegatus) were compared in BAPN-treated and control embryos. These mRNAs accumulated normally during BAPN treatment, even though the embryos did not gastrulate. To determine if the expression of any gene product is sensitive to ECM disruption, a differential cDNA screen compared poly (A+) RNA from BAPN-arrested and control embryos in Lytechinus. A cDNA clone was isolated from this screen that represented a 2.1 kb mRNA that did not accumulate during BAPN treatment. Removal of BAPN resulted in the accumulation of this transcript coincident with the onset of gastrulation. This cDNA clone encodes a L. variegatus homologue of LpS1, recently demonstrated to be an ancestral homologue of the aboral ectoderm-specific Spec 1-Spec 2 gene family in S. purpuratus. Nuclear run-on assays in L. pictus suggested that transcriptional activity of LpS1 was selectively inhibited by BAPN treatment. Thus, although the accumulation of many gene products occurred independently of the embryonic collagenous matrix, the accumulation of LpS1 and LvS1 appeared to be mediated by the ECM.

Antidromic Conduction in a Direct Posterior Pathway from Midbrain to Hippocampus (Part 2 of 2)

An average field response to electrical stimulation in the hippocampus has been recorded in dorsomedial rat midbrain. The fully developed response is triphasic. Latencies of the three components and their behavior during repetitive stimulation are compatible with the assumption that they are antidromic and originate, respectively, in proximal axons, cell bodies and dendrites. Conduction velocity is estimated to be 5 m/s. Reversal in polarity of the response at different recording points within a distance of 1 mm and abolition by small midbrain lesions confirm location of the active neural elements in that region. Lesions in the white matter of posterior cingulum and in the thalamocortical radiation abolish the response. Latencies of the response recorded in the thalamocortical radiation are in accord with the conclusion that the fibers pass in this location. The findings support results of an earlier study suggesting that visual information is transmitted without a synapse from dorsomedial midbrain to dentate gyrus by a pathway that reaches the cerebral hemisphere in the thalamocortical radiation and enters hippocampus from white matter in the posterior cingulum. A study of visual responses in the medial and dorsal wall of the cerebral hemisphere in lizards may help to evaluate the hypothesis that in these regions are the ancestral homologues for dentate gyrus and cornu ammonis, respectively, in mammalian hippocampus.

Antidromic conduction in a direct posterior pathway from midbrain to hippocampus.

An average field response to electrical stimulation in the hippocampus has been recorded in dorsomedial rat midbrain. The fully developed response is triphasic. Latencies of the three components and their behavior during repetitive stimulation are compatible with the assumption that they are antidromic and originate, respectively, in proximal axons, cell bodies and dendrites. Conduction velocity is estimated to be 5 m/s. Reversal in polarity of the response at different recording points within a distance of 1 mm and abolition by small midbrain lesions confirm location of the active neural elements in that region. Lesions in the white matter of posterior cingulum and in the thalamocortical radiation abolish the response. Latencies of the response recorded in the thalamocortical radiation are in accord with the conclusion that the fibers pass in this location. The findings support results of an earlier study suggesting that visual information is transmitted without a synapse from dorsomedial midbrain to dentate gyrus by a pathway that reaches the cerebral hemisphere in the thalamocortical radiation and enters hippocampus from white matter in the posterior cingulum. A study of visual responses in the medial and dorsal wall of the cerebral hemisphere in lizards may help to evaluate the hypothesis that in these regions are the ancestral homologues for dentate gyrus and cornu ammonis, respectively, in mammalian hippocampus.

Isolation of the major low molecular weight gliadins from wheat

The four major components of the low-molecular-weight gliadins (LMWG) from Triticum aestivum L. endosperm have been purified to homogeneity, as judged by electrophoresis at two pH-values. Their amino acid compositions, although typical of prolamins, are different from those of classical gliadins in that they have a higher proportion of Cys and Met and a lower proportion of Glx and Pro. The composition divergence values calculated for the binary comparisons of these components are those expected for proteins with a high degree of sequence homology. These results, which indicate homoeology (ancestral homology) among the genes encoding LMWG, are in agreement with the previous genetic evidence.

A method for detecting distant evolutionary relationships between protein or nucleic acid sequences in the presence of deletions or insertions.

A method for detecting homology between two protein or nucleic acid sequences which require insertions or deletions for optimum alignment has been devised for use with a computer. Sequences are assessed for possible relationship by Monte Carlo methods involving comparisons between the alignment of the real sequences and alignments of randomly scrambled sequences of the same composition as the real sequences, each alignment having the optimum number of gaps. As each gap is successively introduced into a comparison (real or random) a maximum score is determined from the similarity of the aligned residues. From the distribution of the maximum alignment scores of randomly scrambled sequences having the same number of gaps, the percentage of random comparisons having higher scores is determined, and the smallest of these percentage levels for each pair of sequences (real or random) indicates the optimum alignment. The fraction of the comparisons of random sequences having percentage levels at their optimum alignment below that of the real sequence comparison at its optimum estimates the probability that such an alignment might have arisen by chance. Related sequences are detected since their optimum alignment score, by virtue of a contribution from ancestral homology in addition to optimised random considerations, occupies a more extreme position in the appropriate frequency distribution of score than do the majority of optimum scores of randomly scrambled sequences in their appropriate distributions. Application of this 'optimum match' method of sequence comparison shows that the sensitivity of the 'maximum match' method of Needleman and Wunsch (1970) decreases quite dramatically with sequence comparisons which require only a few gaps for a reasonable alignment, or when sequences differ greatly in length. The 'maximum match' method as applied by Barker and Dayhoff (1972) has the additional disadvantage that deletions which have occurred in the longer of two homologous protein sequences further decrease the sensitivity of detection of relationship. The 'constrained match' method of Sankoff and Cedergren (1973) is seen to be misleading since large increments in the alignment score from added gaps do not necessarily result in a high total alignment score required to demonstrate sequence homology.

Relationships among low MW hydrophobic proteins from wheat endosperm

Low MW proteins extractable with chloroform-methanol mixtures from wheat endosperm have been purified from different Triticum species and partially characterized. Their amino acid composition and MWs are consistent with previous genetic evidence concerning relationships among these proteins: proteins CM1 and CM2 are homoeologous (ancestral homologues); proteins CM3 and CM3′ are allelic variants; proteins 16 and 17 are homoeologous.

Primary Structure of Alfalfa Ferredoxin

The amino acid sequences of the six peptides obtained from tryptic digests of alfalfa ferredoxin were determined by chemical and enzymic degradations. The order of tryptic peptides was established from the properties of the two segments of the succinylated protein obtained by tryptic hydrolysis at the single arginyl residue. Alfalfa ferredoxin consists of a polypeptide chain of 97 residues, characterized by clusters of acidic and hydrophobic amino acids. It is suggested that this type of distribution of hydrophobic and strongly hydrophilic residues implies a low α-helical content of the native structure. Of the 5 cysteinyl residues only 2, separated by 2 serine residues, show a grouped arrangement similar to that observed in clostridial ferredoxins. Alfalfa ferredoxin differs from spinach ferredoxin at 19 residue positions. The over-all effect of these substitutions is largely conservative in terms of the total numbers of charged, hydroxy- and hydrophobic residues. Most of the variant positions are clustered in the vicinity of Ala-Ala sequences, leaving several long stretches unaltered. Comparisons of peptide segments based on minimal mutation distances indicate the occurrence of internal repetitions of similar sequences 27 and 14 residues long in alfalfa ferredoxin. A rigorous statistical comparison of alfalfa and clostridial ferredoxins demonstrates a degree of amino acid sequence similarity clearly greater than would be expected on the basis of random occurrences. Although this raises the possibility of a common evolutionary origin for all ferredoxins, without knowledge of the minimal number of invariant residues required for function, such analyses cannot conclusively prove ancestral homology.

Cytogenetical studies in sesamum

Interspecific crosses betweenSesamum orientale Linn, andSesamum prostratum Retz. were effected reciprocally and the sterile hybrid was made fertile by artificial induction of amphidiploidy through colchicine. The cytology of the parents and the hybrids was studied in detail. Details of meiosis ofSesamum orientale, one of the parents employed have been worked out. The peculiar persistence of the nucleus and its movements during the meiotic cycle are recorded. The other parentSesamum prostratum has also been cytologically studied. The irregular meiosis of the sterile hybrid and the occurrence of scattered bivalents and univalents in the metaphase plate, leading to the ultimate formation of abnormal sporads have been described fully. The regular meiosis of the fertile amphidiploid is compared with the irregular meiosis of the sterile hybrid and the cause of this regularity is explained. The nucleolus with behaviour of the special regard to its persistence and movements is discussed. Interspecific hybridisation as a guide to ancestral homology and the artificial synthesis of a new species are discussed in the light of cytological data gathered in the present investigation. The origin of the cultivated TilSesamum orientale Linn. from a putative ancestor having haploid number 7 through allopolyploidy is traced with the help of cytological details obtained in the hybrid meiosis. The origin of the wildSesamum prostratum Retz. is also traced to an ancestral form possessing haploid number of 8 chromosomes through autopolyploidy.