Why did heterospory evolve?

The primitive land plant life cycle featured the production of spores of unimodal size, a condition called homospory. The evolution of bimodal size distributions with small male spores and large female spores, known as heterospory, was an innovation that occurred repeatedly in the history of land plants. The importance of desiccation‐resistant spores for colonization of the land is well known, but the adaptive value of heterospory has never been well established. It was an addition to a sexual life cycle that already involved male and female gametes. Its role as a precursor to the evolution of seeds has received much attention, but this is an evolutionary consequence of heterospory that cannot explain the transition from homospory to heterospory (and the lack of evolutionary reversal from heterospory to homospory). Enforced outcrossing of gametophytes has often been mentioned in connection to heterospory, but we review the shortcomings of this argument as an explanation of the selective advantage of heterospory. Few alternative arguments concerning the selective forces favouring heterospory have been proposed, a paucity of attention that is surprising given the importance of this innovation in land plant evolution. In this review we highlight two ideas that may lead us to a better understanding of why heterospory evolved. First, models of optimal resource allocation – an approach that has been used for decades in evolutionary ecology to help understand parental investment and other life‐history patterns – suggest that an evolutionary increase in spore size could reach a threshold at which small spores yielding small, sperm‐producing gametophytes would return greater fitness per unit of resource investment than would large spores and bisexual gametophytes. With the advent of such microspores, megaspores would evolve under frequency‐dependent selection. This argument can account for the appearance of heterospory in the Devonian, when increasingly tall and complex vegetative communities presented competitive conditions that made large spore size advantageous. Second, heterospory is analogous in many ways to anisogamy. Indeed, heterospory is a kind of re‐invention of anisogamy within the context of a sporophyte‐dominant land plant life cycle. The evolution of anisogamy has been the subject of important theoretical and empirical investigation. Recent work in this area suggests that mate‐encounter dynamics set up selective forces that can drive the evolution of anisogamy. We suggest that similar dispersal and mating dynamics could have underlain spore size differentiation. The two approaches offer predictions that are consistent with currently available data but could be tested far more thoroughly. We hope to re‐establish attention on this neglected aspect of plant evolutionary biology and suggest some paths for empirical investigation.     

Physiological Responses and Partisan Bias: Beyond Self-Reported Measures of Party Identification

People are biased partisans: they tend to agree with policies from political parties they identify with, independent of policy content. Here, we investigate how physiological reactions to political parties shape bias. Using changes in galvanic skin conductance responses to the visual presentation of party logos, we obtained an implicit and physiological measure of the affective arousal associated with political parties. Subsequently, we exposed subjects to classical party cue experiments where the party sponsors of specific policies were experimentally varied. We found that partisan bias only obtains among those exhibiting a strong physiological reaction to the party source; being a self-reported party identifier is not sufficient on its own. This suggests that partisan bias is rooted in implicit, affective reactions.     

Gonadal development and growth in 46,XX and 46,XY individuals with P450scc deficiency (congenital lipoid adrenal hyperplasia).

We have investigated gonadal development and growth in 4 individuals (3 with 46,XY and 1 with 46,XX karyotype) with P450scc deficiency. One patient died at 2 months of age from adrenal insufficiency, while the remaining 3 individuals were healthy and developed normally (age at follow-up: 18, 10 and 8 years). In the surviving individuals, the diagnosis was established during the first 2-4 months of life by extensive endocrine studies of blood and urine. In the remaining patient, the diagnosis was made on the basis of karyotype (46,XY), anatomy of internal and external genitalia and adrenal pathology. Gonadectomy was performed in the 2 surviving 46,XY individuals at the age of 7 years, and histological examination showed normal testicular morphology but very few germ cells. Postmortem examination of the testes of the 2-month-old subject showed normal testicular histology, and quantitative analysis revealed a normal number of germ cells. Ultrasound of the 46,XX individual showed normal internal female genitalia including ovaries with follicles. The 3 surviving patients grew along the 75th (46,XY), the 90th (46,XY) and the 50th percentile (46,XX), respectively. The oldest girl experienced normal breast and pubic hair development after oral estrogen replacement and topical testosterone administration. The glucocorticoid and mineralocorticoid replacement was adjusted in accordance with repeated measurements of serum sodium and serum potassium, plasma renin concentration and blood pressure. No attempts were made to normalize serum ACTH. We conclude that prenatal testicular maturation and development of female internal genitalia may take place in the absence of normal steroid hormone production. Normal growth and development may be obtained in P450scc-deficient individuals with adequate hormone replacement.     

Purification and characterization of a growth inhibitory hepatic peptide. A preliminary note

A pentapeptide isolated from normal mouse liver seems to inhibit DNA synthesis (3H-thymidine incorporation into liver DNA and labeling indices) and the mitotic rate (G2-M cell flux) in regenerating mouse liver. The inhibitor is somewhat similar to the growth inhibitory pentapeptides previously reported for granulocytes and epidermis. It is active at very low dose levels, showing a bell-shaped dose-response curve.     

HOMED: a homologous sequence editor

The alignment of homologous sequences with each other and theirdisplay has proved a–difficult task, despite a frequentrequirement for this process. HOMED enables related sequencesto be edited and listed in parallel with each other. The editorfunction uses a full screen editor which emulates the text editorsKED and EDT (on PDP–11 and VAX–11 respectively)and which can be adapted to emulate other text editors. Thisemulation has been adopted to simplify user learning of editingfunctions. HOMED provides functions for listing the sequencesin a variety of formats and for generating a consensus sequenceas well as providing a series of tools for maintenance of thesequence database. HOMED has been implemented in Pascal in amodular fashion to enhance portability. Received on November 27, 1986; accepted on January 8, 1987